Sue explains LATENCY

"At what point do you raise criteria for a faster response? Before adding the cue?"

Yes. Don't use the word at all until she's absolutely offering you
the behaviour, then start using the word AS SHE'S STARTING TO SIT,
essentially saying "Oh, by the way, that thing you're doing? It's
called SIT". When you've done that maybe a hundred times, try saying
the word to her sometime when she's not thinking of sitting, and see
what happens. If you get a good response, super. If you don't, go
back to naming it as she does it.

The bottom line is, yes, latency is one of the criteria for any
behaviour. If she takes 20 seconds to respond to a cue in agility,
the run is over, thanks for your donation. When you say SIT, you want
to see your word strike her ear, arrow into the brain, and ricochet
to her back end and knock the legs out from under her. If she's
thinking about it, stop using the word, she's not ready yet.

Sue Eh?