When your system becomes suddenly sluggish for no obvious reason, it might be a good idea to check for quarantine problems with installed apps or their helper daemons.
Run Console.app and go into system.log. Check if there are repeating events similar to this one:
May 3 14:36:23 FurloSK-MacBook-Air com.apple.xpc.launchd[1] (com.globaldelight.Boom2Daemon[1770]): Could not find and/or execute program specified by service: 155: Refusing to execute/trust quarantined program/file: com.globaldelight.Boom2Daemon May 3 14:36:23 FurloSK-MacBook-Air com.apple.xpc.launchd[1] (com.globaldelight.Boom2Daemon[1770]): Service exited with abnormal code: 78 May 3 14:36:23 FurloSK-MacBook-Air com.apple.xpc.launchd[1] (com.globaldelight.Boom2Daemon): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
If you can see something like that, it means there is some helper app which the system is trying to run, but since it is quarantined, it is instantly killed after the launch. The problem with such quarantined apps is that the attempt to run them is then repeated every 10 seconds. This means that such apps are repeatedly killed every 10 seconds, and a CrashReport daemon is run every 10 seconds which attempts to analyse the crash. This is the cause of a noticeable slowdown of your system and sometimes also of major free memory jumps.
To solve this, you need to remove the quarantine flag from the target helper app. For this, you need to find out where the helper app resides. It might be listed in one of the *.plist files in the /Library/LaunchAgents or /Library/LaunchDaemons folders. Or it might be inside the regular app, such as in my case – the Boom 2.app contains inside its contents the Boom2Daemon.app. Thus, the solution is this:
xattr -d -r -s com.apple.quarantine /Applications/Boom\ 2.app
(Note that you need to target the whole app, not just the unix program inside the app bundle!)