Nice, this speedup is more than I would have expected. I really enjoy working with postgREST and the many benefits it brings - performance, security, simplification, flexibility.

Do you plan to use the postgREST native filtering via URI params? It can save a tremendous amount of work since each endpoint can have arbitrary filtering without needing to manually write-up the validation and logic for each parameter. I'm sure you already know this. But if you decide to use it, you have to follow particular steps in order for your function to be inlined and to have good performance: check this Q&A with the postgREST devs.

Regardless of whether you go for the filtering or not, since you are using plpgsql functions for the endpoints (just like I was), you can get substantial performance improvements just from switching to pure SQL and making the functions STABLE.