Debug watch builds are slower in angular 7

[HarelM]

Bug Report or Feature Request (mark with an x)

- [x] bug report

Command (mark with an x)

- [ ] new
- [x] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

Versions:

npm: 6.2.0
node: v8.11.3
angular-cli: 7.1.0

Repro steps

Using the following repository:
https://github.com/IsraelHikingMap/Site
Inside IsraelHiking.web run npm install,
npm run build -- --watch
change a file and measure the time it takes to finish incremental build.
Now do the same for the openlayers branch.
Changes in incremental build are significant...

Desired functionality

incremental build time should not change that much between angular 6 and 7.

Mention any other details that might be useful

Overall build time of production is also becoming an issue which got a bit worse in angular 7.

[alan-agius4]

Duplicate of #13102

[angular-automatic-lock-bot]

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