The bottleneck isn't at all the checksum computation itself. It's that to keep checksums valid we need to protect against the potential of torn pages even in cases where it doesn't matter without checksums (i.e. were just individual bits are flipped). That in turn means we need to WAL log changes we don't need to without checksums - which can be painful.

Interesting. I guess M1 doesn't have the 'crc32' "acceleration" that is included in SSE4.2.