WarashiA slow Mac is usually one of four things: an app using too much CPU, not enough memory, heat, or a nearly full disk. Here's how to tell which, in that order.
1. Is one app using the CPU?
Open Activity Monitor (Applications › Utilities), choose the CPU tab and sort by % CPU. On Apple silicon, 100% means one full core, so a single app can show more than 100%. A browser tab, a stuck helper or an indexing service (mds, mediaanalysisd) are the usual suspects. If something sits near the top for minutes while you're not using it, quit it or its tab.
2. Is memory under pressure?
Switch to the Memory tab and look at the Memory Pressure graph at the bottom, not at “Memory Used”. macOS fills free memory with caches on purpose. Green is fine. Yellow means macOS is compressing memory and starting to swap. Red means it's swapping heavily, and that's when everything feels sluggish. See How much RAM do I need?
3. Is the Mac too hot?
When a Mac gets hot, macOS slows the chip down to cool it. A sign of this is kernel_task suddenly using a lot of CPU: it's making room so the chip runs cooler. Read Mac running hot?
4. Is the startup disk nearly full?
macOS needs free space for swap, updates and caches. Below roughly 10–15% free, things slow down. Check › System Settings › General › Storage.
The hard part: it was slow ten minutes ago
Activity Monitor only shows this second. If the slowdown already passed, the evidence is gone. That's the problem a recorder solves: with history, you can scroll back to the moment it happened and see which app was behind it.