Build watch takes 30x as long as cold build, high transformTime

[lingz]

TypeScript Version: 3.7.3 and 3.4.5

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:

Related Issues:

[sheetalkamat]

Can you please provide more details on repro. This seems like its taking long time to do declaration emit but I cant be certain.. Did you try running tsc --d --extendedDiagnostics. What do you see then.

[lingz]

@sheetalkamat the above pasted diagnostics is in fact from extendedDiagnostics and declarations emit only as you suggested. Let me know if any other information could help give clues.