macOS Sequoia is Apple's version 15, released in September 2024. It introduced iPhone Mirroring (the ability to control your iPhone directly from your Mac), an overhauled Passwords app, and tighter continuity features between Apple devices. For most people the upgrade went smoothly. For some, the first few weeks were frustrating: Macs that had been perfectly responsive started stuttering, fans spun up at idle, and apps took seconds longer to open than they had the day before.
The cause was a set of memory leaks in the early Sequoia releases. A memory leak is when software claims memory and then never gives it back, even after you've stopped using the feature that needed it. Over hours or days, this leaves your Mac with less and less free memory, which forces it to work much harder to keep everything running.
If your Mac still feels slow in general, the guide on what to do when your Mac is slow after a macOS update covers the wider picture. For a plain-English explanation of what memory actually means on a Mac, start there before diving in here.
What memory leaks did Sequoia have?
Three issues stood out in the early Sequoia release window, each with a different cause and a different set of people affected.
The WindowServer leak with iPhone Mirroring. WindowServer is the macOS process responsible for drawing everything you see on screen: windows, menus, animations, the desktop itself. In Sequoia 15.0, using iPhone Mirroring caused WindowServer to accumulate memory without releasing it. Open an iPhone Mirroring session, close it, and WindowServer's memory footprint would be larger than before you started. Do that a few times over the course of a day and the total could reach several gigabytes. The problem was significantly worse on Macs with two or more displays connected.
The Mail.app leak in 15.0 and 15.0.1. Mail, the built-in email client, had a separate and unrelated leak. Left running in the background, Mail would gradually consume more and more memory: a process that might normally sit at 200 or 300 MB would swell to multiple gigabytes over several hours. This one was insidious because Mail tends to stay open all day and most people don't monitor its memory use. If your Mac felt fine in the morning and sluggish by the afternoon, this was often why. Apple patched it in Sequoia 15.1.
Spotlight re-indexing on first install. This one is less a bug and more an expected but heavy process. Some users upgrading to Sequoia 15.0 found that Spotlight re-indexed their entire drive from scratch, which can take several hours on a large library and makes the Mac feel genuinely slow during that time. Unlike the leaks above, this is temporary: once indexing completes it doesn't repeat unless you wipe and reinstall.
How to identify which one is affecting you
The fastest way to diagnose memory issues on a Mac is Activity Monitor. Press Command-Space, type "Activity Monitor", press Return, then click the Memory tab at the top of the window.
Look at the Memory Pressure graph at the bottom of the window. Green means your Mac has comfortable headroom. Yellow means it's working harder than ideal. Red means it's genuinely struggling and may be using your disk as a substitute for real memory, which is slow.
If the graph is yellow or red, look up the list for the biggest consumers. If WindowServer is near the top and you've been using iPhone Mirroring, that's the WindowServer leak. If Mail is showing a multi-gigabyte figure and you haven't opened it in a while, that's the Mail leak. For a detailed look at what WindowServer does and when high usage is a problem rather than just normal behaviour, see this guide to WindowServer on Mac.
If neither WindowServer nor Mail is abnormally large, the slowness may be Spotlight still indexing. You can confirm this by looking for mds and mds_stores in the CPU tab of Activity Monitor. If they're using high CPU, indexing is in progress. Give it time.
The fixes that work
Update to the latest Sequoia release. This is the most important step and should come first. Apple shipped fixes across 15.1, 15.2, 15.3, 15.4, and subsequent point releases. The Mail.app leak was addressed in 15.1. The WindowServer issues were progressively improved across the later releases. If you haven't updated recently, go to Apple menu › System Settings › General › Software Update and install whatever's available. This alone resolves most of what's described in this post.
Disable iPhone Mirroring when you're not using it. Even after Apple's patches, closing iPhone Mirroring when you don't need it is good practice. Don't just close the window: right-click the iPhone Mirroring icon in the Dock and choose Quit. A closed window doesn't mean a closed app on a Mac. This is especially worth doing if you have more than one display connected.
Quit Mail and reopen it daily. If you're on an older Sequoia version or notice Mail growing in memory, the quickest fix is to quit Mail fully and reopen it once a day. Press Command-Q while Mail is the active app, or right-click its Dock icon and choose Quit. When Mail reopens it starts fresh. This was the practical workaround during the worst of the 15.0 and 15.0.1 window before the patch landed.
Restart weekly. Memory that accumulates from leaks gets released when you restart your Mac. A weekly restart is a low-effort habit that keeps accumulated junk from building up. macOS prompts you automatically when an update requires a restart, but it's worth doing even on weeks when no update arrives.
Reduce login items. Every app that launches automatically at login consumes memory from the moment you log in. Go to System Settings › General › Login Items and remove anything you don't regularly use. This won't fix a specific leak, but it gives your Mac more headroom to work with, which makes any memory pressure easier to absorb.
Let Spotlight finish. If the slowness arrived immediately after installing Sequoia and your Activity Monitor shows high CPU from mds or mds_stores, the fix is patience. Connecting your Mac to power and leaving it idle overnight usually completes the indexing. Trying to use your Mac heavily while Spotlight indexes makes both tasks slower.
Should you update or upgrade?
If you're still on Sequoia 15.0 or 15.0.1, update urgently. There's no good reason to stay on those versions: the memory leaks are real, the patches exist, and installing the latest 15.x release is straightforward from Software Update. It should take less than half an hour.
If you're already on a recent Sequoia point release and things feel fine, there's no pressure to upgrade to Tahoe. Sequoia is stable in 2026. Apple continues to ship security updates for it, and it will remain supported for another year or two alongside whatever macOS follows Tahoe.
If you're considering upgrading to Tahoe and your Mac is older (Intel, or Apple Silicon with 8 GB of memory), it's worth reading the guide on macOS Tahoe slow performance before committing. Tahoe introduced a much heavier visual design than Sequoia, and on constrained hardware the trade-off may not be worth it for you. The Sequoia memory leak problems from 2024 are done; the Tahoe performance considerations are real and ongoing for certain hardware configurations.
Apple maintains a guide to installing macOS updates if you need a walkthrough of the update process itself.