Have you checked the UniFab GPU setting? I have not seen the same problem you report. When both DVDFab and UniFab are using my AMD GPU, Unifab Converter is at least as fast as DVDFab 12.
The same does not apply to Compressor - my first test with that seems to show big problems with slowdown, even though is hows 60% GPU use.
Same file, same settings, Converter takes less than 3 minutes, Compressor about 15 minutes. But I'm still not sure of what I am seeing. That first test was with MP4 output format, and I am now re-running the same Compressor job with MKV output, and it is running very fast. No way of knowing yet if that first test job was just a fluke, or these numbers will change at random. So far, I think this UniFab product is a terrible unstable mess!
Just for comparison, I have now run 4 tasks in Unifab with this same input file, and 3 of them have run in about 2:45. Now running same task in DVDFab12, and it is running about twice as lone (5 minutes +). So on this computer, UniFab seems much faster, but it has shown a LOT of problems so far, including output files that will not play properly on my TV (still testing that).