Backdrops Set To 'Obstacle' causes more lag than Actives?

I made a level with many backdrops as obstacles, and framerate is 60 if I don't move the character. While moving, it drops to 19. I've made games on this scale using actives instead, and there was no lag. What should I do? Why does backdrops cause more lag than actives? (It doesn't happen if they aren't set to obstacle)

Quote from Yves

You should post an example so that we can see what you do exactly and can profile your app.

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The movement code is in the PLAYER active, but i don't think it's relevant to the problem. It will run fine when you first test it. Set the backdrops (big purple blocks) to obstacle, and then massive lag, but only when the camera is moving.

Generally speaking, which type of object would you recommend I use for laying out the walls of the levels, that would have the fastest performance?

Quote from marbenx

20,000 by 40,000 is quite a large frame size - not sure why the obstacles specifically slow it down but reducing your frame size to even 20,000 by 20,000 (and moving all the objects up so they're still in the frame) keeps it at 60fps for me

I tried that, and the framerate went up by about 10. still nowhere near 60fps. My PC is decent, so I feel like this should be easy for it. I could make mulitple smaller sections of the level, and have transitions bewteen them, and i could probably do 10k x 10k, but that really isn't ideal.

edit: I lowered it to 8000x10000, and am getting 60fps consistently. I want later levels to be larger though, so still looking for other solutions if anyone has any.