The same problem here: After upgrading to 1.8.6 with Big Sur affinity photo has become even more slow and crashes frequently. Sometimes starting the app takes 5 sec, sometimes 30 sec. Very unstable. During working the app seems to slow down gradually. Then, e.g. refining selections will become a pain. Brush actions are visible after 5 sec or more and the refining action is everything but refining. Also sometimes problems with adjustments: Although the results seems to be very good first, after inverting the mask, the brush reveals sometimes anything else but not the result before inverting. All these problems are especially related to HDR merging of raw files. Handling small jpg-files (less than 10 mega) seems to work better.
Also impossible to keep more than 2-3 photo documents open. Working with the third photo will inevitably provoke crashing. Is this related to Affinity photo or Apple who wants me to purchase a clean new expensive computer? iMac (Retina 5K, 27-inch, Late 2014); 3,5 GHz Quad-Core Intel Core i5; memory 24 GB 1600 MHz DDR3, AMD Radeon R9 M290X 2 GB. Other programs work as to this moment well and fast (not yet tried iMovie with Big Sur, iMovie issues have been visible sometimes with only 8 GB memory). I would like to keep my old computer and be content with the slowness if this would be the only problem (exporting to jpg takes sometimes 5 sec, sometimes 30 sec or more, I could live with that).
Thanks.
