Sudden and painfully slow performance on a 2017 27" Mac.

You've installed a couple of the worst apps possible for Macs: MacKeeper and CleanMyMac. Both are considered to be akin to malware by most of the experienced contributors here.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Uninstall MacKeeper and CleanMyMac according to the developer's instructions. Find Any File can help in removing leftover components of apps that have been deleted. 

Do the following:  

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support. 

NOTE: Safe Mode boot can take 2 -3 minutes as it's doing the following:• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

#2 - download and run the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For the two apps mentioned you'd do the following search(es): 

1 - Name contains mackeeper

2 - Name contains cleanmymac

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

#3 - reboot normally.