Item #1: I don’t feel relying on compiler optimisation to have the code execute performantly is the solution. This code worked fine under V7.
Item #2: Two external buffers were used previously in V7 and worked perfectly so there is clearly some significant change in V8 wrt updating/refreshing regions.
BTW If you are not already aware the example projects included with NXP’s SDK are based on V7 and configured to use full size double buffers for all LVGL certified boards from NXP.