The 30 second answer
I start by agreeing on a concrete target as percentiles, for example p99 under a set time at a given request rate. Then I write a load test for that target and run it before each release. In production I watch latency, traffic, errors and saturation with alerts on p99. When we miss the target, I use traces to find where time goes before changing anything.
First I agree on a target as percentiles, for example p99 under 300 ms at a given request rate. I write a load test for it, with k6 or JMeter, and I run it in CI or before each release. In production I track latency, traffic, errors and saturation on dashboards, with alerts on p99. If we miss the target, I use traces to find where the time goes before I change anything.