cargo check is fast. It's only slow when the build goes through (barring extreme use of compile-time proc macros, which is rare and crate-specific).

i mean as a first order approximation context (the key resource that seems to affect quality) doesn't depend on real compilation speed, presumably the agent is suspended and not burning context while waiting for compliation

If you have an LLM that doesn't make errors ever, then you have an ASI, at which point the conversation is meaningless. In the meantime, having a lower error rate but more uncaught errors is less important than making incorrect code impossible to compile, and/or flagged by strict linters.