I track this stuff because...well, I'm me. Based on Apple's marketing claims using the A4 chip in the original iPad as the baseline, below is the performance scaling.
From the A10X to the A12Z: 35% faster CPU and 100% faster graphics
From the A12Z to the M1: 50% faster CPU and 50% faster graphics.
The 50% faster graphics is indeed 50% faster than the already faster A12Z, but it looks like the jump to the A12Z may have been the all-round bigger change in experience. Another factor is storage speed. I know the A12X/Z had increased SSD performance (not sure if marketed though). The M1 is advertised as having 2x the storage. Not sure how the SSD performance will be, but that could make a different for reeder apps loading local content.