HitFilm does not perform as well as some other programs with certain types of footage. When that happens your best option is to transcode to a more edit friendly format. If your footage was recorded with a cell phone, tablet or screen capture software then odds are it was recorded with a Variable Frame Rate (VFR) and the only way to fix it is to transcode it to something with a Constant Frame Rate (CFR). Here's a couple of articles covering the reasons why it's a good idea to transcode regardless of the editor you're using.
Right now there is a lot of information scattered throughout the forum on all the different options available and this thread is an attempt to collect and organize it in one place. Sometimes that will mean making new comments here and other times it will mean linking to existing threads like this example of the kind of performance you can expect after transcoding:
And this long list of available codec options for reference:
For most things your best option is going to be Norman AVC created with Handbrake.
While Norman AVC is a decent option for many things we now have native Cineform support so unless you want or need to keep your file sizes down Cineform is the intermediate format of choice.
Avid DNxHD/HR or Norman AVC are the top choices. Cineform is also a viable option but may not perform as well.
Mac users can ignore most of what's here because you already have full access to ProRes. At some point Mac options for transcoding to ProRes might be added (any volunteers?) for completeness.