Mac running slow after a macOS update

After this, the Mac feels normal again because the background work that was slowing it down has actually finished — not because you've worked around it.

Why this happens

A macOS update triggers a handful of one-time background jobs that are genuinely CPU- and disk-intensive: Spotlight rebuilds its search index for anything the update touched, Photos re-analyzes your library if its database format changed, and iCloud services resync. None of this is unique to Apple Silicon, but SSD speed and available cores affect how long it takes, which is why the same update feels faster on a Mac with more headroom. On a healthy Apple Silicon Mac with an SSD, initial re-indexing commonly wraps up in 15–45 minutes; a full re-index of a large drive can run for a few hours. If it's still churning more than a day after a major release (or more than a few hours after a routine point release), something has likely gone wrong rather than just being slow to finish.

If you're specifically running Intel apps under Rosetta, there's a second, Apple-Silicon-specific mechanism worth knowing: Rosetta caches translated code on disk, and some updates leave that cache in a state where the first launch of a translated app after the update takes noticeably longer while it retranslates. This is separate from the general indexing slowdown and only affects apps that still need Rosetta at all — see whether Rosetta 2 is slowing your Mac down if that's the specific pattern you're seeing. If the CPU stays pegged well beyond any of this and the fans won't quit, that's more likely kernel_task under thermal pressure than an update side-effect.

Fix it

1. Open Activity Monitor (Applications → Utilities), sort by % CPU, and check whether mds_stores, mdworker, or photoanalysisd are the ones actually using it. If so, that's Spotlight or Photos doing its one-time post-update work, not a fault. From Terminal, the same check:

   ● Reads the system

   What it does: lists the top CPU-consuming processes right now.

   After: nothing changes, this only reads.

   ps -Ao pid,pcpu,comm | sort -k2 -rn | head -8

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   If mds_stores or mdworker is near the top, Spotlight is still working. Typical output shape:

     PID %CPU COMM
   12345  85.2 /System/Library/Frameworks/CoreServices.framework/.../mds_stores
   23456  22.1 /System/Library/Frameworks/CoreServices.framework/.../mdworker_shared

2. Confirm Spotlight indexing is actually enabled and running rather than stuck disabled:

   ● Reads the system

   What it does: prints whether Spotlight indexing is enabled for the startup volume.

   After: nothing changes, this only reads. Note: this reports enabled/disabled status only — it does not show live indexing progress, so pair it with the CPU check above rather than relying on it alone.

   mdutil -s /

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   Expected healthy output is Indexing enabled. If it says disabled and you didn't turn it off yourself, that's worth investigating separately — but it means Spotlight isn't your slowdown.

3. If both checks come back clean — nothing unusual using CPU, indexing enabled and presumably finished — give the Mac two or three full restarts before assuming anything is actually wrong. Multiple post-update caches (Rosetta's among them) only get rebuilt properly across an actual restart, not a sleep/wake cycle.

Still broken?

• kernel_task itself is the one using all the CPU, with fans at max
• The update itself never finished installing, or failed partway through

Verified on

Related problems

• kernel_task pegging the CPU with fans at max
• The update itself is stuck or failed to install