I have the same problem on Mojave, even with display set to Metal and parallel processing turned off. This is a real pain, and I frequently convert large batches of images from one icc profile to another.
I successfully installed the Nik plug-ins and they seem to work. However, in photoshop the plugins duplicate the selected layer and add a layer mask so that the changes are non-destructive. That doesn't seem to happen in Affinity Photo. Am I missing something?