3.6 regression: composite project build time is slower than in 3.5.3

[dbrockman]

tsc --build in a composite project is a lot slower in version 3.6 than in 3.5.3.

I compared version 3.5.3 with version 3.6.2 and typescript@next (the next tag resolves to 3.7.0-dev.20190830 at the time of writing).

When I do a clean build, without any *.tsbuildinfo files, it takes about 65 seconds on all these versions.

When running the build step again, with the *.tsbuildinfo files, it takes about 6 seconds on version 3.5.3 and about 55 seconds on version 3.6.2 and typescript@next.

version	clean build time	repeated build time
3.5.3	~ 65 sec	~ 6 sec
3.6.2	~ 65 sec	~ 55 sec
@next	~ 65 sec	~ 55 sec

[AnyhowStep]

I've experienced this Heisenbug a bunch of times.

I've had moments where a clean build would take 20s,
but an incremental build (by modifying one line of code) would take 80s with 3.5.1

Whenever that happens, I just do a clean build.
Then, subsequent incremental builds take a short amount of time again.

I can never seem to reliably reproduce this behaviour.

[dbrockman]

I should add that I ran the build step 3-4 times on each version and got very consistent results. The build time only differed by < 1 sec.

[sandersn]

@dbrockman can you share your project? If you can share it privately, contact @sheetalkamat. When you've done that, unassign yourself so that we know to take a look at this.

@AnyhowStep if you find a reliable repro, we would appreciate that too.

[chrisnojima]

This happens to me also. you can checkout our project here
https://github.com/keybase/client
Master is 3.5 and incremental builds take ~7 seconds
keybase/client#19190
This is a PR for 3.6 and incremental builds take ~120s

[sheetalkamat]

Can you please try build from #33145 (comment) to see if its the same issue?

[sheetalkamat]

@chrisnojima I was able to repro with your instructions and #33415 does not help it. I am looking into it.