After reading lots about this problem, here the solution for Hitfilm PRO 2017source try to use Handbrake to get an mp4 with anamorphic auto, modulus 2, codec h.264 (x264) constant frame rate, encoder preset in slower and constant quality between 20 and 22, convert
Once you have your mp4 this way, convert to GoPro Cineform using Hitfilm, Quality Medium.A 1 Gb mp4 will now be an 18 Gb avi aprox (1 hour to convert)
Load your Cineform avi into Hitfilm, almost zero lag fwd and rev, no need for proxy or cached ram. (well, depends on your video size, 720, 1080, 4K) the difference in response is huge using Cineform.
Why? Hitfilm PRO 2017 now has native encoder/decoder for GoPro Cineform.VLC player requires a plugin for Cineform codec. (Not needed at all since the idea is not to reproduce it, just edit it in Hitfilm)