Elementary, my dear Watson...
Just open WindowsTask Manager and set the priority of Videopad process(es) to Low (tab Details in Windows 10).
The CPU will be 100%, however the Videopad processes will be performed at lower priority and you will be able to do normally everything else on the same computer.
It would be nice if the process x264enc7.exe would be able to use the GPU engine, then the Export video file process would be much faster. At the latest Videopad versions there was added Options -> Editing -> Use accelerated Video effects, however it does not use GPU engine at exporting the video file.