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Why is my Mac slow and hot: how to find the cause

October 2026 • 7 min read

Open Activity Monitor and look at two tabs: CPU and Memory. The first shows which app loads the processor, the second shows whether you have enough memory. If both look calm, check the free space on your disk. Apple builds the search for a cause the same way: its article "If your Mac runs slowly" names a startup disk without enough free space, an app that needs more memory than your Mac has available, an app that is not compatible with your Mac, apps you are not using, and problems with the disk itself (Apple: If your Mac runs slowly).

Step 1: find what loads the CPU

  1. Press Command-Space, start typing "Activity Monitor" and open the app (Apple: Find what you need with Spotlight).
  2. Go to the CPU tab.
  3. Click the % CPU column heading so the busiest processes come to the top. The arrow in the heading shows the sort order.
  4. Look at the top rows and at the bottom of the window. It shows three values: System is the share of CPU capability used by processes that belong to macOS, User is the share used by apps you opened and the processes they started, Idle is the share that is not used (Apple: View CPU activity in Activity Monitor). If Idle is close to zero, the processor really is busy.

The percentage in the column is counted per core, so a process with several threads can show more than 100. In Terminal, a process with three busy threads showed 268% in top. Apple does not say whether the column in the Activity Monitor window behaves the same way, and I did not check it in the window.

What to do with what you see at the top:

At the top What it is and what to do
An ordinary app with high usage Quit it. If you need to continue the work, pause it
kernel_task Part of macOS that cools the processor. Look for what heats your Mac: kernel_task high CPU
mds, mds_stores, mdworker_shared Spotlight indexing, which can run for hours after an update: mds_stores high CPU
WindowServer The system part that draws windows: WindowServer high CPU
Helper (Renderer) A browser tab or a web view inside an app: Chrome Helper high CPU
No clear leader, little Idle Several apps share the load. Quit the ones you are not using, which is what Apple advises

Step 2: check memory

  1. In Activity Monitor, go to the Memory tab.
  2. Look at the Memory Pressure graph at the bottom of the window. Apple explains the colors this way: green means your computer is using all of its RAM efficiently, yellow means it might eventually need more RAM, red means it needs more RAM (Apple: Check if your Mac needs more RAM in Activity Monitor).
  3. If the graph is yellow or red, sort the list by the Memory column and find the app that takes the most. Apple notes that an app you are using may require more memory than your Mac has easily available.
  4. Do not be alarmed by a big number in Memory Used. Apple writes that having free or unused memory does not necessarily improve performance.

For yellow and red, Apple's RAM article gives no recipe. It only suggests checking whether you can add memory to this Mac: hold Option and open System Information from the Apple menu, choose Hardware, then Memory, and look at the Memory Slots pane. If the pane is not there, your Mac does not support memory upgrades.

If you prefer Terminal: memory_pressure with no arguments prints memory statistics and ends with a line "System-wide memory free percentage". On my Mac it showed 33%. Apple does not say how this number relates to the color of the graph, so read the color in Activity Monitor.

How to tell a memory shortage from CPU load

It is the CPU It is memory
Where to look CPU tab, Idle at the bottom Memory tab, Memory Pressure graph
Sign Idle is close to zero, one or two apps at the top The graph is yellow or red
What helps Quit the app or pause it: a pause removes CPU load, from 100% to 0% in the measurement in the pause article Quit or restart the app: a pause does not free memory
More How to pause a process How to pause a process, the memory section

Both problems can happen at once, so check both tabs, not one.

Step 3: check the disk

If the CPU is idle and memory is fine, look at the disk. Apple names two causes: the startup disk may not have enough free space, or there may be problems with the disk itself. For the first, the article suggests moving files to another disk or external storage and deleting files you no longer need, and it points to the storage recommendations in System Settings under General > Storage. For the second, it suggests verifying the disk in Disk Utility and trying to fix any issues.

If the slowdown started after a macOS update

I did not test this on Tahoe: my Mac runs macOS 15.7.7. I did not find an Apple article about Tahoe slowing a Mac down. I found something else: Apple writes that Spotlight indexing happens after you install a software update or upgrade, and that depending on the amount of data it can take hours or even days (Apple: About Spotlight indexing and search results). So in the first days after an update, check the mds and mdworker_shared processes: how to do it. Keep the Mac plugged in and do not interrupt indexing: Apple says it finishes sooner when the device is idle, plugged into power, and connected to Wi-Fi or Ethernet.

I found nothing in Apple's documentation about other background work after an update, including photo analysis, so I will not claim it.

What to quit and what not to

  • Fine: apps you opened yourself and recognize. Quit them the normal way first, with Command-Q: Apple advises this because a normal quit lets the app save changes (Apple: How to force an app to quit on Mac).
  • With care: Force Quit in Activity Monitor. The process quits immediately, open files may lose data, and other apps or processes that depend on it could have problems (Apple: Quit an app or process in Activity Monitor). To quit a process there, select it, click the Stop button in the toolbar and choose Quit or Force Quit. That button quits the process; it does not pause it.
  • Leave alone: processes whose names you do not know, and system ones: kernel_task, WindowServer, launchd. According to its man page, WindowServer is a system daemon launched by launchd and is not user-invoked.

How this relates to Aide

Aide watches CPU and memory and lists the apps that went over the limit you set. The indicator in the menu bar changes color, a click opens the list, and each row has Pause and Kill buttons. So the first step of this article, finding the culprit in a process list, comes down to a glance at the indicator. Aide does not show macOS system processes, does not cool your Mac, does not watch the disk, and does not replace checking memory with Apple's graph. More on the home page.

FAQ

Why is my Mac slow and hot at the same time? When an app loads the CPU hard, the chip heats up. Apple describes that in this case kernel_task makes the CPU less available to the processes that use it most intensely, to bring the temperature down (Apple: kernel_task). Find the app on the CPU tab.

Will a restart help? Apple's article "If your Mac runs slowly" does not suggest restarting. A restart quits every app, so the load disappears for a while, but it does not tell you the cause.

Do I need to buy more memory? Not right away. Apple advises looking at the Memory Pressure graph first and checking whether memory can be added to your Mac at all.

More on specific processes

Sources

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