Slower compilation when using @ngtools/webpack@7.1.0

Initially reported in https://github.com/angular/angular/issues/26674#issuecomment-443263655 by @sod

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hm indeed, just updating ngtools gives me the slowdown. Also using angular 7.1 with old ngtools 7.0.7 is significantly faster.

versions	1st run	2nd run	3rd run	4th run	5th run
@angular 7.0.4, @ngtools 7.0.7	31s	15s	7s	5s	5s
@angular 7.0.4, @ngtools 7.1.0	29s	14.5s	15.5s	12s	12s
@angular 7.1.1, @ngtools 7.1.0	29s	13.5s	13.5s	15s	16s
@angular 7.1.1, @ngtools 7.0.7	28s	12.8s	7s	5s	4.2s

[clydin]

The issue is indeed that within 7.1.x the Angular compiler is always used to calculate the set of lazy routes when in JIT mode. This was initially done to guarantee that the lazy routes were calculated accurately and any lazy route related errors would be shown upon a rebuild. This has the unfortunate downside of taking more time. The underlying problem is one of correctness/accuracy versus speed.

The team has decided to bring back the faster but potentially less accurate method of detecting lazy routes upon JIT rebuilds (first builds will always use the more complete Angular compiler method). Applications that do not have lazy routes within libraries and that only use direct string literals with loadChildren should not be affected by the potential of less accurate detection. Note that the function overload of loadChildren also does not apply to this situation. For those projects where correctness of lazy route detection outweighs rebuild speed, please consider using AOT mode for development. AOT mode will also provide a full set of template errors as well which JIT mode is not capable of doing.

If you have concerns about this decision, please let us know.

[clydin]

@psurrey It already has been. Ensure that the project is using both @angular/cli@7.2.1 and @angular-devkit/build-angular@0.12.1.