Every single signature hashes a few kilobytes of data, on average, and there are ~10k signatures per block (more with segwit..). That's 10's of megabytes of has

Every single signature hashes a few kilobytes of data, on average, and there are ~10k signatures per block (more with segwit..). That's 10's of megabytes of hashing per block, even without considering adversarial transactions that purposefully inflate those numbers. That's in the range of 10-100 MB. On my CPU openssl benchmarks about 180MB/s for sha256, so 50-500ms of hash time per block. That's not insignificant.