New build is not my thing and might not be yours, just for a different perspective.
I know a few new build/site Plumbers. When they have worked for me, they are not much quicker than me when doing the type of work I do. In fact when on commercial work they are slower through lack of experience or because we might be using different materials than they're used to, which is understandable.
As mentioned above, repetition is a factor if all houses are the same or similar.
As Simon said, study & thought save time. Make a brew, stand and look at the job. If you've done it in your head, you'll see the best way to do it which will save time.
I can't stand things not being plumb, level or square and that dictates my speed. Some have a view that if a man on a galloping horse thinks its straight then that'll do! This is another factor sometimes on sites.
