Load or cost: the first fork in the road

Something got slower. Only two things can have happened

Either more work arrived, or the same work started costing more. Almost every incident is one or the other, and they lead to completely different places.

The distinguishing evidence is on screen the whole time and takes ten seconds to read: request rate and utilisation. If they climbed, it is load. If they did not, it is cost — and the number of requests has nothing to do with it.

A service that is slower and no busier did not get more work. It got more expensive work, or it started waiting for something.