First of all, you can clean up the hard drive or update/reinstall OS. Sometimes it's enough to uninstall software you don’t use and use cleaner apps. But make sure that you are using only the best mac system cleaner.
1. Replace your Mac hard drive with an SSD
Moving from a traditional spinning hard drive to a solid-state drive (SSD) is the single best thing you can do to improve the performance of an aging MacBook
2. Add more memory (RAM)
While you have your MacBook opened to replace its hard drive, take the opportunity to add more memory.
3. Clean your Mac's hard drive
Uninstall old Mac apps
Clean up applications you still use
Find out which apps are using the most resources
Delete big, unused files
4. Reduce login items
If your Mac is slow to boot up, the problem may be that there are too applications to open at startup. It's likely you never set them to launch at startup -- they launch by default.
5. Keep current with OS X
Hope this help!
I highly recommend you use CleanMyMac 3 of Macpaw. It's a great app, you can check out reviews here: https://omghowto.com/the-cleanmymac-3-review-should-you-buy.html