Do application leftovers have any impact on performance? Apparently, uninstalling an app by dragging it to the bin may leave application leftovers in some folders (such as Library/Preferences/, Library/Applications Support/, Library/StartupItems/ etc...). Several sites explain how removing such leftovers can increase disk space.

However, I have no problem with storage. So, this is my question: apart from taking up disk space, do these application leftovers have any negative 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.