Whenever i put a 720p 60fps video file that is longer than 20 minutes, my Hitfilm slows down heavily, is it because of my cpu (i3-3250), or my ram (8gb)? I want to know where the problem comes from because it's really annoying.
I can't speak to the specifics of your CPU, though the i3 is on the low end of the compatibility list. Your RAM should be sufficient.
On a general note, HitFilm works much more smoothly with shorter videos. If you can split your long piece into smaller ones, that would help. I recommend clips no longer than maybe 5-6 minutes each.
Transcoding can also help. If you don't know what that is, then you're probably not doing it, and I strongly recommend watching this video for more information.
Long story short: if you transcode your single long clip into several shorter ones, you'll probably have a much smoother editing experience, even with a less-than-ideal CPU.
It looks like you're new here. If you want to get involved, click one of these buttons!