I’ve run into the same thing on my Mac after a couple of years. It’s usually not one big issue, but a bunch of small ones adding up.
First thing I’d check is Activity Monitor (Applications → Utilities). Look at the CPU and Memory tabs. If you see something constantly eating high CPU or memory, that’s often the culprit.
Browsers (especially Chrome with many tabs) are common offenders.
Also:
Restart your Mac if you haven’t. This sounds basic, but it clears a lot of temporary junk
Check your startup items (System Settings > General > Login Items) and remove unnecessary ones
Make sure you have at least 15–20% free storage. macOS slows down a lot when the disk is nearly full
Update macOS. Sometimes lag comes from bugs that get fixed later
If it only gets slow after hours of use, it might be memory pressure building up or some background process leaking resources.
Since RAM is vital for the performance of the Mac, I’ve installed a Mac cleaner - iBoysoft Cleaner.
This tool supports releasing the RAM with a click. Moreover, it helps clean up junk files on your Mac with AI smart scanning and analyzes Mac space with AI recommendations.
As for me, my Mac keeps running faster with the assistance of this tiny tool. Accelerating a Mac is a long-term task. So, I think a good Mac cleaner matters a lot.