Why would you want to slow the system down by clearing cache?
Program startup gets noticeably faster once its cached and clearing cache forces the system to reload programs from flash memory instead of RAM, which is slower.

Only slowdowns I ever have are either caused by restarting the phone (indexing and loading from flash) or stuff that seems I/O related (updating feeds).

I usually have uptimes of >100 days and performance is as good if not better than first day. Only thing needed IMO is keeping track of possible runaway/crashed processes. Programs don't always exit cleanly and can get stuck (not freeing the memory claimed by them). In that case kill command is your friend.