MP4 is a highly compressed keyframe based format, and it's indeed not recommended to use with Resolume.
It's being decoding on the CPU, so playing multiple 4k mp4 files at once can have a big hit on your CPU usage. Basically it can only be used, played back at nominal speed, and forwards.
On a NUC I could imagine you're running into CPU limits playing the files, as you're also running into with encoding, and rendering.
Encoding a 45 minutes 4k file to DXV with Alley on my Theradripper desktop takes about 30 minutes.
We recommend to use DXV encoded video clips with uncompressed audio for best performance. DXV can be decoded on the GPU, taking the load off your CPU.
Resolume installers also install a plugin for Quicktime, and Adobe products, so DXV should also show up as an option in the Quicktime formats, or stand alone in the Format list - depending on which converter you use - so you can export right to DXV.
If you're working between mac and PC, you could also try prores, but that's also decoded on the CPU, and the file sizes are also huge.