How compute intensive is a CA, anyway? I’d think that a few Raspberry Pi-ish devices would be enough to do it for an entire enterprise (or maybe even one, but fault tolerance).
In general I would assume when you shorten certificate validity from 90-ish days to one the computational resources required for issuance will increase to about 90x of what it was before. So it would be a lot more expensive to run.
Also, unrelated to that argument but I think the most expensive part in a CA is not issuance but rather OCSP since an OCSP query could happen with every TLS connection (although stapling exists to solve that). OCSP would not be affected by a shortening of validity periods.
