It looks like you were right the first time around in a way! So I had a few versions of the background video (that were definitely paused in the Media Player and were not placed in the level), and after deleting them the framerate jumped up to almost 70!
So it looks like media in your content browser, even if it’s not playing in the media player, is somehow impacting performance while the level is running.
Do you know if this is a bug or if there is a potential fix? I’m currently working on one level with one BG video, but I’ll be implementing different levels with different BG videos and it’ll be a pain if videos that are not present in that level are impacting performance this much.