That's (a) just one objective and (b) too vaguely stated. What's an acceptable p95 latency for a FaaS? For a database write? For a message queue? The answer is a very big "it depends". And why are we only talking about latency?
There are tradeoffs literally everywhere you look in this space, and not knowing enough about the application's performance, reliability, and efficiency goals can sometimes be a real hindrance to being good at running that application.
Until something falls over and you haven't deployed any new code and all the vendor's systems are green. Part of the reason you went with a PaaS in the first place is that you didn't want to manage the infra yourself, you just want to ship application code.
This has worked great so far and so you've stopped thinking about the infrastructure at all. Disk and network I/O still exist though, you've just been incentivized to stop thinking about them and so all you know is that the vendor sucks, when in reality it's the application's fault.
Somebody's gotta roll up their sleeves and see what's going on in there though, so hopefully your PaaS vendor is as cool as we are :)
More broadly, almost any application these days is "just API calls". The issue is which API calls, how many, with what frequency and where. That essentially is the application, and a PaaS employee doesn't magically know that stuff instinctively.
You're missing the point. A PaaS, or serverless service doesn't need to know your application. That's the whole point. They're just API calls, and they need to succeed with consistently low latency.