Typically, TypeScript type-checks `lib.d.ts` every single compilation which can add at least half a second of build time. It will also do a full check of `.d.ts` files in your dependencies, which can also be pretty heavy.

We're considering ways we can avoid a full type check: https://github.com/microsoft/TypeScript/issues/31417

In the meantime, you can try out turning on the `skipLibCheck` compiler option to see if that helps at all, but be warned that it might not catch conflicts across multiple files.