TypeScript performance during watch mode

[deftomat]

CRA performance can still be a pain with TypeScript.

We already reduce the waiting times for type-check by #5903, then we speed it up even more by using the IncrementalAPI in fork-ts-checker-webpack-plugin.

Unfortunately, even after all of these improvements, type-check can took 5 or more seconds to finish in big projects. Yes, it's much better now as you can see your changes immediately and type-errors will appear when they are ready.

Now, the important thing:

I discover that using the isolatedModules: false in tsconfig.json reduces type-checks between code changes from 5 seconds to less than 1 second.

I know the reason, why we are using isolatedModules - it is a limitation of Babel compiler. However, we should definitely do something with it as developers are required to wait a lot of time just to catch a few edge cases.

I'm not sure how to solve it right now but I'm pretty sure that we can figure it out together.

cc @johnnyreilly (creator of fork-ts-checker-webpack-plugin)

[johnnyreilly]

Credit where it's due: @piotr-oles is the creator of fork-ts-checker-webpack-plugin - I've just worked on it an awful lot. And I'm louder 😁

In ts-loader we did fiddling with isolatedModules. I've actually forgotten most of the details. However here's some links that should provide some useful info:

microsoft/TypeScript#16351

TypeStrong/ts-loader#569

[Bnaya]

I'm curious: if you run standalone tsc -w --notEmit,
How much time does the typecheck takes? (with and without isolatedModules )

[deftomat]

@Bnaya The same happens for tsc -w --noEmit. So, its definitely the behaviour of TypeScript itself.

Looks like isolatedModules has some serious performance implications.

[deftomat]

Looks like it is a bug in TypeScript itself: microsoft/TypeScript#32294

[deftomat]

Closing as fix will probably be available in TypeScript v3.7.