Came here looking to see if others were having the same problem. Glad to see not just me. Affinity Photo is laggy with the OpenCL option enabled. Using something like the clone brush tool, the image update lags behind the brush cursor. I have turned off OpenCL acceleration. It's not a big deal, the program was plenty responsive enough before the option, so I just disabled it on my Windows installs. But it was the only reason I upgraded to version 1.9 (and went through the Windows updates to get the new feature available).
Computer #1
Laptop with i7-10657 + Intel Iris Plus Graphics
Computer #2
Desktop with Ryzen 1700 + AMD RX460
I'm perfectly happy with the performance with the option turned off, but just wanted to add more data points.