WinDirStat on Windows, I am not at Windows machine right now but IIRC the original WinDirStat walks the whole filesystem, can take long time; faster forks of it use the NTFS Master File Table (MFT), which more like just looking up into already collected by the system.
(I used OmniWeb as my favorite web browser for NeXTStep, followed it to macOS. Also their pro diagramming tool, OmniGraffle - remember Visio before Microsoft bought it?)
https://www.idealsoftware.com/opensource/fast-windirstat.htm...
On the Mac, I've used Omni Disk Sweeper for pro a the last 20 years, and it still receives updates. Not open source, but no $$.
https://www.omnigroup.com/more/
(I used OmniWeb as my favorite web browser for NeXTStep, followed it to macOS. Also their pro diagramming tool, OmniGraffle - remember Visio before Microsoft bought it?)