I made the mistake of right-clicking a script and asking the editor to show me what referenced it in the scene; it caused Unity to hang for 3 minutes straight, and to hang for 3 minutes every time I did anything at all until I dismissed the “ref” search in the Hierarchy search bar.
This feature either needs a massive performance boost or it should be deprecated.
This doesn’t solve all cases but you can actually type the class name in the hierarchy window, and if you add a t: to it, it will only return instances of that class, instant-filtered:
t:PlayerController
For ransacking an entire project I tend to right click show in folder on the C# file, then get the meta file GUID and use a rapid find-in-files to see where it might be used, which only shows me the files, not WHERE in the files, but once I know the files, I can open them and use the first technique.