Well, that's because of the way the mechanics work. Things slow down when there are not enough resources available. Of course I can make it skip the frames in order to stop going "slow-mo" but then I don't really know how well various thigs would work. Still, worry not! More optimization to come! Thanks for the comment!
then dont use unity. 2d games should run fast on computers from 10 years ago (think pentium 2).
if you can find away to make it run faster in unity, then your good enough to do it even in c++ and smfl/alegro etc.
from my experience this can be anything, do you render all the level? maybe dont render the parts that are off screen. this can be any number of small things.
Frankly, I think some of it (the lag) is there because of the code; it could be the fact that it's not that resource-efficient. I have to work more on it. This is one of my main priorities right now.