Hi, I heard recently that this can happen with Nvidia cards if you have G-sync turned on.
That's too bad. It could be that an anti-virus or something is scanning it in realtime and slowing it down. I created this game in the same engine as Nuclear Throne, Undertale/Delta-Rune and Hotline Miami 1 and I understand that the same problem occasionally affects those games too. Searching for solutions to the same problem in those games might help anyone still struggling.