> Verifying checkmate is essentially doing a 2-ply search: for all moves i can make, the opponent has a reply that captures my king. This is more work than verifying normal moves, where a 1-ply search suffices to check that kings are not left in check.

I was going to make a counter-argument but then I realized that this system doesn't normally enumerate legal moves, simply verify legality of moves that are actually played, which is a crucial distinction here.

Thanks for making this observation; I found it a pretty subtle point!