In my experience, startup time increases with MP's databases becoming larger, especially Moving Pictures can take ages to load. Same goes for loading a large saved music playlist on startup.
I remember it was discussed a while ago whether to load plugins in the background and/or in parallel?
Background loading may cause other issues as you need to sync when a plugin is actually completed and block the user hitting it until then otherwise you may get issues with the semi loaded plugin. This would require a change in MP and updated to the plugin interface for things like IsLoaded etc.
Perhaps guidance on how to make the main process light and information on how to use Background Worker calls, Async/Await and general synchronization methods so plugin authors can optimize their own experience.