I know exactly what you’ve done. in Project Settings/Game Settings you’ve enabled ‘Fixed Frame Rate’. Don’t do this. I think that this forces the game to render 90 frames (or whatever) per second, and if the engine can’t update at that rate then it makes a second take longer if you see what I mean. Imagine in 3dsmax or something you’re making a 30 second animation at 25fps. It will take a lot longer than 30 seconds to render but you get 30x25 perfect frames.
What you might want to do instead is alter the framerate cap in the console (t.MaxFPS) (it might be locked at 60, but you can unlock it or set it to 90 - although it sounds like your game can’t perform at this speed).
Dan