Just a check, you haven’t limited FPS with a console command?
Generally speaking, I don’t really know exactly what the system is doing when you’re manipulating BPs, but it always knows when you want to add an interface method, or a custom event from another place. So, I’m guessing that it’s looking stuff up the whole time.
If you have one HUMUNGOUS BP, and it has to run through that every time you drag something, it is going to get very slow.
Make sure you don’t have auto save enabled, or make a copy of the project before doing this, but I’m wondering if you delete the level with that mega BP in it, does everything speed up again.
If so, it’s because there’s no decoupling in the project, but we’ll come back to that.