I just ran it for the first time... I've run windirstat many times including in the last two weeks and yes, this is very fast. According to its timing it took 3.63 seconds to scan my drive... I'm not sure if it really took almost 3 minutes for windirstat, but it did take a long time, so it's plausible. More importantly, ~ 4 seconds is much easier to wait for.

It is - it ‘cheats’ by reading off the NTFS MFT, similarly to another indispensable piece of Windows software Everything Search, both of which I miss keenly on my underprovisioned work Mac.