It might seem obvious that you'd want to transcode your footage to a more "editing friendly" format so that editing is faster - Cineform, Norman AVC, DNxHD, etc. - but it also has other benefits.
I don't know why I didn't think of this before, but I took an .MP4 file and just exported it and noted the time it took. I then transcoded it to .MP4 again, to the same quality/bitrate, but adjusted so it would run faster in the editor.
The first clip was a little jerky when playing at full resolution, but the second one was as smooth as butter when I dragged the playhead forwards and backwards (which is more work to decode than forwards).
Doing nothing else to it at all, I then exported it with the same settings as the first clip. It took less than half the time to export. I loaded both clips back in and zoomed in to detailed areas to examine them and the quality of both clips was identical. Being able to load the clips in faster meant Hitfilm was able to write them out again faster too. Yay!
So, if you're going to be exporting multiple revisions of something: it makes sense to help Hitfilm get those files in faster, to get them out faster.
Don't just struggle along with jerky files when editing, as everything else (presumably even RAM Preview and Proxy creation, although I didn't time those) and making 'test renders' to see the result at a decent speed will all be slower. Double bad news.
But...double good news if you transcode first.