My understanding is that completion in VS Code for C# is provided by Omnisharp, which yeah, I agree, seems to be slow and inconsistent.
I've written a language server extension for VS Code (not for C#), and the completion lists I provide are better than those I get in C#/F# while writing the server, it's bad :/
The screenshot I posted a while back showed me exporting something at the of the file and I had typed `Det` at which point it suggested some random functions from random APIs. Then I had `Detail` at which point no suggestions were offered. At `Detaile` it finally gave me `DetailedProgress` from the same file about 50 lines higher up.
This is a frequently recurring theme that it takes so long to actually suggest mere names (~ strings).