Registrations don't cause a lot of load. They're essentially just one row in a table. Certificates have to be signed by a Hardware Security Module with limited

Registrations don't cause a lot of load. They're essentially just one row in a table. Certificates have to be signed by a Hardware Security Module with limited capacity. OCSP messages have to be signed every couple of days for the lifetime of a cert by the same HSM. This is significantly harder (and more expensive) to scale.