If you have a slow PC like me, but want a smooth playback in Hitfilm, there is a solution.
1. Compress your files with Handbrake
Compress them as much as possible. Quality doesn't matter since we won't export these in the end. We only need a smaller file size to be able to edit smoothly. Save these smaller files in a different folder, but with the same name as the original.
2. Import your large files into Hitfilm
Import them and put them into timeline in correct order.
3. Relink footage
In the media section, right click on one of the large video files, navigate to the compressed file and relink. Do this process for all files.
4. Edit your video
Edit smaller files as usual.
5. Relink again
Relink files from media tab like before, but this time you are relinking smaller files to original files. Everything you edited will apply to your original files.
Export your film and you are done!
I hope this helps.