Improve Find All References performance?

Our testing indicated that performance was on par with Visual Studio and that VS had some bugs that was causing worse performance (due to it getting stuck processing on 1 thread some times) -- @Colengms do you know the VS bug that you filed?

But...there's probably stuff we could do to improve performance, but since the bottleneck is generally IntelliSense parsing, I'm not sure yet if the performance gain would be worth the effort.

Generally, the "too long" case will occur if there are too many files to "confirm" -- users could use the cancel or preview if they don't want to wait for confirmation.

Let us know if anyone has a specific repro where you believe we're doing something incorrect (i.e. performance is slower than with VS, similarly configured) that is causing bad performance or too low CPU usage, e.g. if you have a 200k line file somewhere in your code base, it could get "stuck" lexing on 1 core.

The "searching" phase could also take slightly longer (than VS) for opened files because we don't re-use the already opened document object so we have to read it from disk.

Also, let us know if anyone encounters too slow "canceling" or too slow "previewing".

No file, test, or concrete reproduction is named. Start by obtaining a specific slow or incorrect Find All References case and compare its searching, confirmation, cancellation, or preview behavior with a similarly configured Visual Studio setup; done requires a reproducible bottleneck and a measurable improvement.