Boot time to desktop is the number everyone quotes and almost nobody experiences. The interval that determines whether a machine feels fast is the one between the desktop appearing and the machine actually responding to input.
The six-layer digest covers how this is separated out in the UX-ROM model, and the layers 4-7 digest covers the middle of that stack in more detail. Six-layer UX-ROM digest.
The measurement harness itself is documented separately, which matters because a latency claim is only as good as the instrument that produced it. Windows latency surface profiler.