If your Bitdefender is having problems
If you're having problems with several programs (I.e. Notepad, Command Prompt) taking a long time to load, as well as overall system sluggishness, and you just got the Windows 2018 update, the issue is probably that Windows did something to turn on security settings, and Bitdefender didn't turn them back off, which causes the two to fight each other. Disabling Bitdefender got rid of most of the sluggishness.
It might be something for the Bitdefender techs to look into and issue a patch to fix.
Until then, if the sluggish behavior can be fixed by uninstalling Bitdefender (I used the dedicated uninstall tool found here) and reinstalling it. Apparently this causes Bitdefender to change the settings back so it has control of security. Whatever reason was making it happen, an uninstall/reinstall fixed it: programs started opening normally again, and other slowness went away.