I have reverted to 6.2.1 because 6.3.3 at startup when not connected to the internet is noticeably slower, sometimes 5 seconds before usable. Is it possibly checking for updates/ and is it possible to disable this check. I couldn't see anyone else reporting a slowdown. I've never had a problem before this latest release.
You may try to disable autoupdate at Settings->Preferences->MISC.->Enable Notepad++ Autoupdater.