Question about water pressure in relation to valve and feeder pipe diameters...

[Bob F]

If flushing the toilet is the problem, an obvious solution would be to partially close the valve to the toilet so it fills slower.

Bob

[Ken Moiarty]

You sound skeptical if my claim. What parts of the country have you lived, if I may ask? Have you lived in custom built houses or production built (or both)? I have no doubt that you have never experienced this while showering, or even while washing your hands for that matter. After all, just because my experience has not been the same is not sufficient cause to doubt what you report as having been your experience.

Ken