Based on the graph the fast function runtime is really short. You might be just seeing effects of efficiency vs performance cores. Lower thread count makes most

Based on the graph the fast function runtime is really short. You might be just seeing effects of efficiency vs performance cores. Lower thread count makes most of the stuff run on performance cores and task end times align more nicely. With larger number of threads tasks running on performance cores complete first and you are left waiting for efficiency tasks running on efficiency cores to complete, or context switches have to be done and tasks are moved between cores which causes overhead. You could try what happens if you have 10 times more images when running fast function. Also you have just 8 physical performance cores and 4 physical efficiency cores. Performance cores have hyper threading so they act as 2 logical cores but that doesn't mean that they can actually execute 2 threads at maximum performance. If processing tasks use the same parts of the processor core, then processor cannot run both threads at the same time and IPC will suffer. Slow task maybe uses more varied parts of the core which allows better IPC with hyper threading. So that also may reduce optimal thread count.