It basically maxes out one ivy bridge ~3.5Ghz core to 90-100% any time anything happens (not so much in early game I guess) (@edit again, maybe 90% was probably an exaggeration most of the time, it's how it looks once things start slowing down - 50-70% is more normal without many enemies of screen), especially large amount of projectiles. Doesn't really touch more cores which I guess is the only way to go without getting deeper into programming afaik, probably nothing to be done about that in the engine? Not sure how to tell if it stresses RAM, as far as the amount goes the limitation from 32 bit limit (and crashes) hits long before my actual 16 GB could get allocated. Speed is 1600 ddr3 in dual channel.
There also the quirk that the game gets slower instead of stuttery as the framerate goes down (so 30 fps is basically unplayable), but again I suppose that's just par for the course with the engine.
A small nitpick that is basically irrelevant but I can't unsee - when you start the game you go through the cloud right, but you can very briefly see the even top of the normal clouds and empty space above it.
I think I may have run into something odd when getting the first weapon, I was clicking my mouse and whatnot beforehand, and found I just couldn't shoot, like the button locked itself. Don't know if it's related to earlier controls issues or something.
Small reminder because I don't know how serious it actually is, but in the second boss (fighters) I had two or three times more had the last one just hide somewhere with next to no health, couldn't find it, couldn't progress. Maybe if you hit it for just enough that it goes down to very low damage it disappears/becomes effectively dead but doesn't trigger the end, or something, I don't know, I'm just guessing.
Also, went into 720p/windowed mode to keep an eye on the task manager and had trouble going back into 1080p auto, auto became 720p basically. Restarted and fixed itself.