Complete newbie question: Do all CRMs charge for 'API access' or is that special?

Precisely. The term "feature tax" perfectly describes the mechanism. It's functionally identical to an access fee for a toll road that's in worse repair than the free public highway next to it.

One related procurement tactic I've used is to frame the benchmark data as a security concern. "Given the higher latency and inconsistency you've measured, our compliance team is questioning if this API endpoint meets our SLA requirements for data synchronization. Could we discuss a service credit or a waiver while these performance gaps are addressed?" It reframes the conversation from "we want a discount" to "your product isn't meeting the bar," which often gets a more favorable response.