I think if you are getting bad slowdown in the editor something may have gone wrong with your save, my level (
link has tons of decals and a few screens with 6+ active objects and I never noticed any slowdown (on vita).
Have you tried
a. Quiting the game/restarting the vita completely?
b. Starting a new level and see if you still have slowdown?
Regarding trimming extra frames, it shouldn't be necessary, a few mostly empty frames aren't causing your problem. Also, notes that are collected will play for the next three screens, so in the level editor you hear the notes on every screen within three of the current screen. If you're making a level and want to get away from your old notes (like if you want to change the tune completely) I think you need to use one of the transport objects like a pipe or an elevator to teleport several screens away.
Hope that helps