Update imports takes too long to show on large projects

[OliverJAsh]

This is a duplicate of #65112, which was supposedly fixed/closed, however this still appears to be an issue for the large project I'm currently working on.

(Since that issue is now locked, I am forced to open a new issue here.)

VS Code 1.32.3
TypeScript 3.3.3333

/cc @samijaber

[mjbvz]

I see this in the VS Code codebase too. Perf seems to have regressed at some point.

Opened microsoft/TypeScript#30535 to track this upstream