So update, moving into HDR fixes the problem for me, however, I drop below 25FPS. Also, looking outside the AC, this tiling is restricted in the top third of the view (screen).
I only have 8GB RAM, and the SMP panel tells me that I have 990MB of video and 205MB of System RAM free.
Does anyone have any graphics settings I should eb using on my iMac 27" 2019:
Model Name: iMac
Model Identifier: iMac19,1
Processor Name: Intel Core i5
Processor Speed: 3.7 GHz
Number of Processors: 1
Total Number of Cores: 6
L2 Cache (per Core): 256 KB
L3 Cache: 9 MB
Memory: 8 GB
Graphics Chipset Model: Radeon Pro 580X
Type: GPU
Bus: PCIe
PCIe Lane Width: x16
VRAM (Total): 8 GB
I appreciate this.