I'm not an expert of legalities around accessibility, but taking the ADA as an inspiration, IIRC the Captcha provider does not have to provide an accessible alternative that works equivalently, only one that allows the same outcome for users with accessibility needs.

(I've seen this explained based on accounts of a public library built in the US which had stairs, but no elevators, for a specific section - this was still ADA compliant as they had staff on hand to serve readers in that specific section)

So a 30s delay or whatever it takes to limit spammers is probably not entirely unthinkable.