Mac

Mac running slow after the macOS Tahoe update

Why your Mac feels slow, laggy, or runs out of memory after updating to macOS Tahoe (macOS 26) — the two confirmed Tahoe bugs with exact fixes, plus the normal post-update slowdown that clears on its own.

Problem summary

I'm here because my Mac got noticeably slower after I updated to macOS Tahoe (macOS 26) — it lags, beachballs, the fans spin up, or I get "Your system has run out of application memory" even with apps I'd normally run fine. Some of this is a real, confirmed Tahoe bug (actually two separate ones) with specific fixes; some of it is the normal post-update reindexing that settles in a day or two. This page tells you which is which and exactly what to do for each, without installing a 'cleaner' app.

Operator snapshotEvidence first

First proof

Check how long it's been since the update and watch Activity Monitor → CPU for mdworker_shared / mds.

Screen to open

defaults write -g NSAutoFillHeuristicControllerEnabled -bool false

Expected signal

Within a day of updating, Spotlight/iCloud reindexing is using CPU and will stop on its own.

Stop boundary

Don't strip the default silently — record that you applied it.

Layer path

1A 'slow after the update' Mac is really three different problems wearing the same symptom: normal post-update background work (Spotlight reindex, iCloud resync) that self-clears, a confirmed macOS 26 autofill bug that makes apps slow down over time, and a separate Electron/WindowServer GPU leak. The fix depends on which one you have, so the first job is to tell them apart.

2The deciding evidence is timing and shape: heavy CPU in the first day = reindexing; getting slower the longer the Mac runs = the autofill leak; fans and lag whenever Chrome/VS Code/Slack are open = the Electron leak; high WindowServer with external displays = Liquid Glass compositing load.

3Memory is judged by pressure, not by the 'Memory Used' number. macOS uses free RAM for cache and swaps by design, so the Memory Pressure graph (green/yellow/red) is the real signal — and the Tahoe leaks can make a healthy-RAM Mac look starved.

Runbook

Step-by-step runbook

Start here. Do each check in order, compare it to the expected result, and stop when the evidence explains the failure or the safe stop point applies.

Rule out reindexing

Check: If it's been under a day, leave the Mac idle and plugged in and watch mdworker in Activity Monitor.

Expected result: Reindex/resync finishes and speed returns on its own.

If not: If it's still slow after a day, continue to the bug fixes.

Clean restart

Check: Restart and uncheck 'Reopen windows when logging back in'.

Expected result: You can tell whether the slowdown builds over time or is there from boot.

If not: If slow from boot with nothing open, jump to the WindowServer step.

Apply the autofill workaround

Check: Run the NSAutoFillHeuristicControllerEnabled `defaults write` and relaunch apps.

Expected result: Progressive slowdown / out-of-memory stops.

If not: Note the change so you can revert it later.

Safe stop: Don't strip the default silently — record that you applied it.

Update Electron apps

Check: Update Chrome, VS Code, Slack, Cursor, Brave to fixed builds; quit any not yet updated.

Expected result: WindowServer/GPU load from those apps drops and the system stops lagging.

If not: If no update exists yet, keep the app quit (Cmd+Q) until one ships.

Cut WindowServer load

Check: Turn on Reduce Transparency + Reduce Motion; restart if WindowServer RAM has grown.

Expected result: Compositing load drops and responsiveness improves, especially with external displays.

If not: If it climbs again over days, log out/in to reset it.

Judge memory by pressure

Check: Read Activity Monitor → Memory Pressure (green/yellow/red), not 'Memory Used'.

Expected result: Green confirms RAM is fine; the cause was a leak or reindex.

If not: Only consider more RAM if red persists under heavy real work with no leak.

Decision tree

If: It's been less than a day since the update and CPU is high from mdworker/mds.

Then: Normal post-update Spotlight/iCloud reindexing.

Action: Leave the Mac plugged in and idle for a few hours to a day; it clears itself.

If: Apps get slower the longer they run, or you see 'out of application memory'.

Then: The macOS 26 autofill-heuristic bug.

Action: Run `defaults write -g NSAutoFillHeuristicControllerEnabled -bool false` and relaunch apps.

Safe stop: Note that you applied it so you can revert with `defaults delete` if autofill misbehaves.

If: Fans spin and the whole system lags whenever Chrome/VS Code/Slack are open.

Then: The separate Electron `_cornerMask` GPU leak through WindowServer.

Action: Update those apps to fixed Electron builds (v38.2.0 / v37.6.0 / v36.9.2+); quit any not yet updated.

If: WindowServer is high, especially with external displays or long uptime.

Then: Liquid Glass compositing load and/or the WindowServer uptime leak.

Action: Turn on Reduce Transparency + Reduce Motion; log out/in or restart to reset WindowServer.

If: Memory Pressure is red under genuinely heavy real work with no leak present.

Then: An actual RAM ceiling for that workload.

Action: Only then consider more RAM — after ruling out the leaks and reindex above.

Evidence

Evidence table

Symptom	Evidence to collect	Likely layer	Next action
High CPU and slowness for the first day after updating.	Activity Monitor shows mdworker_shared / mds; Spotlight menu says 'Indexing'.	Normal post-update reindex/resync.	Leave it idle and plugged in; re-test after it finishes.
Apps balloon in memory over hours; 'out of application memory'.	One process grows steadily in Activity Monitor → Memory; quitting it frees the RAM.	macOS 26 autofill-heuristic bug.	Apply the NSAutoFillHeuristicControllerEnabled workaround and relaunch.
Fans loud and system laggy with Electron apps open.	WindowServer + the Electron app show high GPU/CPU; quitting the app calms it.	Electron _cornerMask GPU leak.	Update the Electron apps to fixed builds; quit un-updated ones.
Sluggish UI with external monitors, WindowServer high.	WindowServer CPU/RAM high; worse with transparency and many windows.	Liquid Glass compositing + WindowServer uptime leak.	Reduce Transparency; restart to reset WindowServer.
'Slow' but Memory Pressure is green and CPU is idle.	Activity Monitor shows no runaway process and green pressure.	Not a memory or CPU problem — possibly disk-near-full or a specific app.	Check free disk space and the specific slow app rather than buying RAM.

Reference

Commands and settings paths

Apply the autofill-bug workaround

defaults write -g NSAutoFillHeuristicControllerEnabled -bool false

Where: Terminal on the Mac, then fully quit and relaunch the affected apps.

Expected: Apps stop progressively slowing down / running out of application memory.

Failure means: If apps still balloon, the cause is the Electron leak, not this bug.

Safe next step: Update Electron apps; revert this with `defaults delete -g NSAutoFillHeuristicControllerEnabled` if autofill misbehaves.

Revert the autofill workaround

defaults delete -g NSAutoFillHeuristicControllerEnabled

Where: Terminal on the Mac, then restart, if you ever need native autofill back.

Expected: The global autofill heuristic returns to its default behavior.

Failure means: If the slowdown returns after reverting, the autofill bug was the cause.

Safe next step: Re-apply the workaround, or wait for a point release that fixes it.

Force a clean Spotlight rebuild if reindex is stuck

sudo mdutil -E / ; sudo mdutil -i on /

Where: Terminal on the Mac, only if Spotlight has been pinning CPU for days.

Expected: The index is erased and rebuilt cleanly; CPU settles once it finishes.

Failure means: If a specific folder keeps stalling it, add it to Spotlight Privacy then remove it.

Safe next step: Let the rebuild finish (it's CPU-heavy briefly) before judging speed.

Confirm WindowServer/GPU load rather than guessing

sudo powermetrics --samplers cpu_power,gpu_power -i 1000

Where: Terminal on the Mac, while the lag is happening.

Expected: Shows real CPU/GPU power draw so you can confirm WindowServer/Electron is the load.

Failure means: If GPU load is low, the lag is elsewhere (disk, a specific app, or pressure).

Safe next step: Match the finding to the matching fix above instead of changing everything.

Hardware boundary

Hardware and platform boundary

Change only when

Consider more RAM only after the Tahoe leaks and reindex are ruled out and Memory Pressure stays red under genuinely heavy real work.
If a near-full SSD is the real drag, free space (or size up storage) before assuming the Mac itself is too slow.

Evidence that matters

The Memory Pressure graph (green/yellow/red) — the real signal for whether RAM is the limit.
The exact macOS point release, since the leak bugs come and go across 26.x.
Your app versions — updated Electron apps remove a major source of Tahoe lag.

Evidence that does not matter

The raw 'Memory Used' number — macOS deliberately uses free RAM for cache.
Swap size on its own — some swap is normal and not a fault.
A cleaner app's 'freed memory' figure — it's marketing, not a diagnosis.

Avoid

Installing memory-cleaner / RAM-booster apps — they purge caches macOS manages and can worsen performance.
Editing the IOPlatformPluginFamily thermal kext to 'fix' kernel_task — it can break thermal protection.
Assuming hardware failure before ruling out the OS and app bugs above.

Related tool/checklist

Use the linked tool when you need a guided plan from your exact symptoms instead of a static checklist.

Device setup troubleshooter

Last reviewed

2026-06-02 · Reviewed by HomeTechOps. Reviewed against Apple's macOS update notes and kernel_task guidance plus Eclectic Light's slow/hot-Mac analysis and 2026 reports of the two distinct Tahoe slowdown bugs; separates normal post-update reindexing from the autofill and Electron leaks, and judges memory by pressure rather than the 'Memory Used' number.

Sources/assumptions

Assumes a Mac on macOS Tahoe (macOS 26) that slowed down after the update; the two named bugs are based on 2026 community and developer reports and may be partially addressed in later point releases — re-verify against current Apple update notes.
The autofill `defaults` workaround and the Electron fixed-version numbers are community-sourced; treat them as current-to-mid-2026 and reversible.
Memory Pressure guidance follows Apple's documented Activity Monitor behavior; swap use alone is not treated as a fault.

Source-backed checks

HomeTechOps turns official docs and conservative safety rules into a shorter runbook. These links are the source trail for the page direction.

Apple Support: macOS Tahoe 26 — what's included in the updates (KB 122868)Authoritative per-point-release version + date list for macOS Tahoe 26.x; cited to pin which point release a Time Machine / Wi-Fi / dock symptom maps to.Apple Support: If kernel_task uses a large amount of CPU on your MacUsed for the rule that kernel_task is a thermal-management symptom (it consumes CPU to help cool the machine), not a bug to kill — find the real heat-generating process instead.Eclectic Light (Howard Oakley): Dealing with slow, hot MacsUsed for the operator approach to a slow/hot Mac — kernel_task is a symptom; do not edit the IOPlatformPluginFamily kext (it can break thermal protection); root-cause the heat source.

Diagnostic rule

1. Check the simplest signal first.
2. Change one thing at a time.
3. Avoid unsafe electrical or data-loss steps.
4. Stop when the evidence says the risk moved.

FAQ

Why is my Mac slow right after updating to macOS Tahoe?

Some of it is normal: after any macOS update, Spotlight reindexes and iCloud resyncs, which hammers CPU and disk for a few hours up to a day or two, then stops. If it's still slow after that — or it gets slower the longer the Mac is on — you're likely hitting one of two confirmed Tahoe bugs (the autofill slowdown and the Electron GPU leak), both of which have specific fixes.

How do I fix "Your system has run out of application memory" on Tahoe?

macOS 26 has a bug where apps slow down and balloon in memory over time. The community workaround is to run `defaults write -g NSAutoFillHeuristicControllerEnabled -bool false` in Terminal and relaunch your apps (revert with `defaults delete -g NSAutoFillHeuristicControllerEnabled` if needed). Separately, update Electron apps like Chrome and VS Code, which have their own Tahoe memory/GPU leak.

Should I buy a memory-cleaner app to fix this?

No. These slowdowns are OS and app bugs, not 'junk files'. Cleaner/RAM-booster apps purge caches macOS would manage on its own and often make things worse. Use the built-in Activity Monitor Memory Pressure graph to judge whether RAM is actually the problem, and apply the specific fixes above.

Does my Mac just need more RAM?

Probably not, if the slowdown started with the Tahoe update. Check Activity Monitor → Memory → the Memory Pressure graph: green means you have enough RAM. Only sustained red pressure under genuinely heavy real work — with no leak present — points to needing more memory. The Tahoe leak bugs can make a healthy-RAM Mac look starved.