A “hot path” is a code path that is perf-critical, either because it’s something that is really perf-intensive, or because it’s latency-sensitive. An example of the former is a long for loop, or large nested iteration that does some core computation and takes a long time. An example of the latter is the measure/layout/draw logic for your UI, which must be as fast as possible to avoid dropping frames. In general, these are code paths you want that to take the least amount of time possible, for one reason or another.