It's not. The functionality gap between VSC and Fleet has further widened, and its future looks even more unsure now they backed out the Kotlin native stuff back to the main IDEs.
I don't think a service account vs an API key would improve performance in any meaningful way. I doubt the AI endpoint is authenticating the API key against a central database every request, it will most certainly be cached against a service key in the same AZ or whatever GCP call it.