i am also facing issues since the update to mojave. Affinity Photo is nearly impossible to use. It hangs on every interaction and needs just a lot of time to do anything, even on small operations like change the brightness. As by now i tried to compose a few HDR projects, develop some RAW files and panoramas. Nothing worked as fast as i know it from before the update. I had to abort the HDR compositions because it was just impossible to do anything. Developing 3 RAW files took like 3 hours, just because i really needed thoose. Panorama worked but a lot less quick then before.
I have a 2017 13inch MacPro (16GB) and never really performance issues with AP or AD.
Also i discovered that there is process that starts with AP, the MTLCompilerService and take always something between 40 and 70% CPU usage till i close AP completly. There is another thread with maybe a similiar problem here:
I don't know which more information you need. i Hope this helps already.