My 2 cents?
In my long experience as an employee of an AASP with customers coming in with 'slow' machines. . . . .
In general, slow performance as it is noticed and complained about by users is caused by (in order of frequency in which I encounter it)
1. Corrupted filesystem.
2. Dying hard drives on older machines.
3. Virus protection software.
4. Exotics like a rogue background process, weird, out of date or corrupted driver/app thing.
5. A hundred other edge cases you encounter 1 or two times a year. Edge cases includes bad sata cables, bad seating of the data cable, strange component failure on a motherboard, aftermarket hard drive weird incompatibilities, minor permission problems and malware (which we don't see much) The autostart worm was more of an issue in the pre-macosx days than anything we have seen in the last decade.
If you have to ask this question, the answer is simple. Keep your data backed up. Don't mess with maintenance unless something breaks.
I clean out my safari cache about once a year. It is only something I do if I notice a performance issue.
Don't worry about your mac unless something isn't working right.