Gunnthorian

I've been facing the same issue with my new Macbook Pro, thankfully I found a fix for me an wanted to share it: after changing "Display" to OpenGL Basic, "Retina Rendering" to Automatic, and turning off "Hardware Acceleration" I found Affinity Photo to be running just as it should. Change your settings to these, see if it fixes anything, then change each one as you'd like until it slows back down again and just revert the previous change. Let me know if this helps!