Troubleshooting Slugglish Post-desktop Startup
First thing I would do is isolate the problem to either your user account or your OS install. To this end, just create a new user and try logging in as that user after a fresh boot. If the system is sluggish also under this account, at least part of the issue must be outside your main user account.

From the "moronic clean-up-your-mac for idiots guides", I suppose, you have already learnt about LaunchAgents and LaunchDaemons, both for individual users in ~/Library/LaunchAgents, and for the entire system, in /Library/Launch{Agents,Daemons} and cleaned those out?

Bear in mind that applications are really trying to be helpful in adding those back whenever you launch them.

As an alternative to purging the files, you could try disabling them via launchctl unload -w ~/Library/LaunchAgents/com.example.launchdaemeon in Terminal, because this adds them to a launchd blacklist that applications will have a harder time to override. From man launchctl:

 unload [-w] [-S sessiontype] [-D domain] paths ...
          Unload the specified configuration files or directories of con-
          figuration files.  This will also stop the job if it is running.

          -w       Overrides the Disabled key and sets it to true. In pre-
                   vious versions, this option would modify the configura-
                   tion file. Now the state of the Disabled key is stored
                   elsewhere on-disk.


Finally, some really archane applications add their daemons to /Library/Preferences/com.apple.loginwindow.plist. You can open this file in Xcode and check if it has any entries that you want to get rid of.