Mac running like a snail after 15 minutes
Here are some ideas for troubleshooting a slow Mac


Open the console app (it is located in the utilities directory). Watch the log for any strange behavior.
Hold the shift key while booting to boot into safe mode, see if the issue continues. 
Turn off half of the programs in the startup, see if issue persists, then turn off the other half. Keep repeating until you find the program that is the culprit. 
Try creating a new user account
Lastly, backup to time machine, and format.