Checks some things:
RAM? Use memtest86+
Logs? Run: journalctl -b -x -p 3
Firmware and microcode? Run: dmesg | grep -i firmware, and run: dmesg -i microcode.
Temperatures? Run: sensors
BIOS/UEFI updated? Check settings to get them optimal.
Drive health? Run: smartctl -a /dev/sda
Kernel issue? Revert to earlier kernel to check differences in behaviour.
Compare running in text mode to GUI. Is graphics implicated?
Are the latest installations implicated?
The above advice is great but the most common thing I run into when I hear somebody complain the system is running slow is a physical issue. You should rule out that the problem is the hard drive itself. Especially if it is a mechanical drive and not a solid state. Use S.M.A.R.T. to look for seek errors on the drive or any other errors corrected or not. Any number over zero may indicate a physical drive issue. If the drive is having errors it takes time for the drive to attempt to correct it and that translates to slow computer to you the user. the S.M.A.R.T. also has a self test feature. use the extended test and let it go. Look at the results and if you have any errors showing then it is time to replace the drive. A solid state or pcie nvme or m.2 drive will speed things up drastically.
I like to rule out physical issues before I go looking at configurations.