Free:

• clean fans and heatsink - others mentioned, and the reason is better cooling so it doesn't throttle
• kill unnecessary services - that's why reinstalling works
• install Linux - not reasonable for everyone, but Linux uses far fewer resources
• delete old files - as disks get full, it takes longer to find somewhere for files to go; try to leave 10-20% free
• try a small overclock - many older CPUs can give a little more without upgrading cooling; only do it if temps look good

Relatively cheap (<$200 each):

• upgrade drive to NVMe - huge difference if running an HDD, still noticeable of running a SATA SSD
• add more RAM (only if you're constantly running out)
• upgrade CPU - esp if AMD since they release lots of CPUs for the same socket

It really depends on what's making it slow though.