RAM, pagefile and render times
Troubleshooting my own issues and found a solution so I thought I'd share in case it helps anyone!
I recently updated my video card on my home system so out of curiosity I was watching the GPU utilization while rendering a scene (~10 3d models and 10 lights) in hitfilm. What I noticed was every 1 or 2 minutes, GPU utilization would drop to zero for a good 30 to 60 seconds and the UI would lock up before eventually getting back to work only to repeat itself over and over again. So I fired up resource monitor and found the culprit... Pagefile swapping! Turns out that 16 GB is not enough RAM and as a result every few minutes it would run out of RAM causing hundreds of page faults (pagefile swapping) which would in turn stall the rendering process until swapping was complete. This was obviously extending my render times significantly. As a work around I found that rendering to image sequence helped a lot and pagefile swapping during rendering would happen very rarely if ever and the render times were shorter! Today my new ram kit arrived (32 GB) and I'm happy to report that all the issues I have mentioned here are gone!
TLDR - If you have slow render problems and the UI is locking up periodically during rendering and you have less than 32 GB RAM, you might want to consider upgrading your RAM or rendering to image sequence if at all possible.
I know the minimum requirement for Hitfilm is 4GB of RAM but if the pagefile thrashing I experienced at 16GB was bad enough for me to investigate it must be unbearable with only 4GB!