Apple - Do application leftovers have any impact on performance?

Application preferences and caches will not have any impact on system performance, but if there are leftover startup daemons running, etc., there could absolutely be a performance penalty. There is no way to know for certain how much of an impact there could be. The best advice I can offer is to always check the application developer's uninstall guidelines. Some applications utilize uninstaller applications to ensure all the application support files are removed.

Files in the following subfolders of the root level /Library or User Library will not affect the performance of your Mac at all, as they will not be read or executed:

  • Application Support
  • Caches
  • Containers
  • Preferences

(Not an exhaustive list.)

Files in the following subfolders might affect performance as they may be loading or trying to load non-existent files:

  • StartupItems
  • Extensions
  • LaunchAgents
  • LaunchDaemons

(Again, not an exhaustive list.)