<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with the latest published version. It may have already been fixed. For npm: `typescript@next` This is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly --> **TypeScript Version:** 3.7.3 and 3.4.5 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** transformTime, Watch, Slow compilation **Code** When building our source code, we can do a cold build in about 25 seconds, but a watch build takes 15 minutes to start on Typescript 3.4.5. On Typescript 3.7.3, the watch time doubles to 30 minutes (60x as slow). Running extended diagnostics shows that the vast majority of the time (96%) is spent on "transformTime". Is there any way to speed this up, as I do not consider our project too large on the grander scale (600k LOC). One thing that was causing slow builds that I already fixed and ruled out was the inlined type definitions bug mentioned in #34119, which cut my build time by a third. But even after this, I still get a very slow `transformTime`. ``` Files: 3043 Lines: 637432 Nodes: 2372135 Identifiers: 850259 Symbols: 1167934 Types: 462989 Memory used: 2036527K I/O Read time: 1.03s Parse time: 2.81s Program time: 6.12s Bind time: 3.05s Check time: 11.54s transformTime time: 473.05s commentTime time: 0.14s I/O Write time: 0.43s printTime time: 474.27s Emit time: 474.27s Total time: 494.99s ``` **Expected behavior:** Watch build is slightly slower than cold build **Actual behavior:** Watch build is significantly slower **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> **Related Issues:** <!-- Did you find other bugs that looked similar? -->