Why is my Mac slow? How to see what uses memory and the processor
When a Mac feels slow, it's rarely the Mac itself. Usually one app is busy, or the apps you have open need more memory than the Mac has, so macOS keeps moving memory to and from the disk.
The short answer: open Activity Monitor (Applications › Utilities). In the Memory tab, look at the Memory Pressure graph and Swap Used. In the CPU tab, sort by % CPU. Quit what uses the most if you don't need it. A high "Memory Used" figure on its own is normal.
What does memory pressure mean?
It says whether apps get the memory they ask for. Green is fine. Yellow means macOS is compressing memory to make room. Red means it is writing memory to the disk, called swap, and the Mac slows down. If it's often yellow or red, quit apps or browser tabs you don't need, or restart.
Is high memory use bad on a Mac?
No. macOS fills unused memory with recently used files on purpose, so they open faster, and gives that memory back as soon as an app needs it. Memory Used is high on almost every Mac. Pressure and swap are what tell you there's a problem.
Why does an app use more than 100% CPU?
100% is one processor core busy all the time. An app working on several cores at once, such as Xcode building or a video export, can show 400% or more. That's fine while it works; an app that stays high while idle is worth quitting and opening again.
Do memory cleaners make a Mac faster?
No. Forcing memory empty, which is what sudo purge and most "memory cleaner" apps do, only throws away cached files that macOS then reads from the disk again. The Mac is slower right after, not faster.
Which Terminal commands show the same?
memory_pressure | tail -1
sysctl vm.swapusage
top -o cpu -n 10 -l 1Two maintenance commands help in rare cases: sudo mdutil -E / rebuilds the Spotlight index when search misses files (it takes hours), and sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder flushes the DNS cache when websites stop opening by name. Most Macs never need either.
Can a full disk slow down a Mac?
Yes. macOS needs free space for swap, caches and updates. With only a few gigabytes free, everything gets slower, and updates may not install. Keeping 10 to 20 percent of the disk free is a good rule.