[bostik]

While I agree with parts of the above, there are bits that I disagree with. It's true that you cannot control the network conditions for third parties, and therefore can never be in a position where you would guarantee an SLA for round-trip experience. But I object the notion that tracking end-to-end latency is useless. After all, the three Nielsen usability thresholds are all about latency(!)

Funnily enough, looking through your itemisation I spot two groups that would each benefit from their own kinds of latency monitoring. End location and internet connectivity of the client go into the first. Third-party providers go into the second.

For the first, you'd need to have your own probes reporting from the most actively used networks and locations around the world - that would give you a view into the round-trip latency per major network path. For the second, you'd want to track the time spent between the steps that you control - which in turn would give you a good view into the latency-inducing behaviour of the different third-party providers. Neither are SLA material but they certainly would be useful during enterprise contract negotiations. (Shooting impossible demands down by showing hard data tends to fend off even the most obstinate objections.)

User-agent and bespoke integrations/workflows are entirely out of your hands, and I agree it's useless to try to measure latency for them specifically.

Disclaimer: I have worked with systems where the internal authX roundtrip has to complete within 1ms, and the corresponding client-facing side has to complete its response within 3ms.