how can I improve my computer’s performance without upgrading hardware?

My computer feels slower lately, and I don’t want to spend money on new parts right now. I’ve tried basic things like deleting files and closing background apps, but it still lags sometimes.

Are there advanced tips or settings tweaks that can help boost performance without upgrading hardware?
What’s the best way to diagnose hidden issues that might be slowing my system down?

Would appreciate any detailed advice or tools you recommend!