Ok I did a little more play test on low settings. In this case graphics were never the issue. Rather how the objects in the memory are handled is the issue. So the issue is with one of your background. Suggested improvements for the first level:
1. While starting the level don't instantiate all the objects on fly. Load them async before user clicks the "START.exe" file.
2. Once loaded don't destroy them just hide them and reset their properties. You can check out object pooling, if not implemented.
3. Regarding the main issue, one of your backdrops in first level spawns a lot of overlapping objects. When that backdrop is active game kind of freezes to 2 FPS. When that backdrop is destroyed the game returns to 60 FPS. The background I am talking about is the this one:
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i think i've identified the problem, one of the backgounds is just 10 line rederers using a suspiciosly high amount of resources, i dont have the code right now but it may be a for() function Inside another for() action, making it use 9 times more resources than actually needed, IF this is the problem, ill upload a patch in about 1 and a half hours, if not, i'll try to identify the porblem, it's definetly that background you showed in the screenshot, not the window pop ups