Before doing maintenance on a computer, make sure you have a backup of your data. Actually verify the most recent files you worked on show up in your backed up data. I will typically do a full backup to an external hard drive separate from any regularly scheduled backups I do before doing any major maintenance or an OS reinstall.
Here are some articles I found from manufacturer or reputable sites that talk about Mac maintenance & troubleshooting.
What to do before you sell, give away, or trade in your Mac
Slow MacBook Air or Pro? Here’s 5 ways to speed up your Mac
What maintenance do I need to do on my iMac
If you have any additional questions, you can always give me a call at (855) 520-5200. Thanks and good luck getting the Mac back up to full speed.