It depends on your speed requirements and whether calls are being sent async or not. Also keep in mind that even with internal apis, an api call is usually multiple network boundaries (service1 --> service2 (potential DNS lookup) --> WAF/security proxy --> Firewall --> Load balancer --> SSL handshake --> server/container firewall --> server/container). Then you get into whether the service you're calling calls other apis etc. You can quickly burn 50ms or more with multiple hops. If you're trying to return responses within 200ms you now have very little margin.
Acknowledging that there are indeed many hops, I think it might be a bit disingenuous to say 50ms is easy to burn, depending on what p-value we're talking about.
IIRC, a typical round trip service call at my current place of work (gRPC with protobufs, vault/ssl for verification, consul for dns, etc) carries a p99 minimum latency (i.e. returning a constant) of around 2ms.
A cold roundtrip obviously takes longer (because DNS, ssl, etc).
It depends on how many 9's you want within 10ms, but there are various simple tricks (transparent to the application developer) that a platform team can apply to get you there.
As a sidenote on calling other APIs, my anecdata suggests that most companies microservice call graphs are at most 3-4 services deep, with the vast majority being 1-2 services deep.
This doesn't show the call graph, but it does demonstrate how many companies end up building a handful of services that gatekeep the core data models, and the rest simply compose over those services: https://twitter.com/adrianco/status/441883572618948608/photo...
