Well you have a lot of restriction through mixer valve and filler as both for main pressure,however there is mains pressure and mains pressure as we know,from 1 bar to 3.5 bar,I know you get higher but then p/reducing valves come into effect on many fittings.I ran from mixer valve in wall to bath filler in 22mm improving any possible restriction,reducing at fittings
Most of the time customer using shower maybe using bath every now and then,so very acceptable fill ,they were happy,I did not do a flow rate,that would change each job anyway
And as water temp is coming through to bath at correct temp anyway through thermostatic mixer,you can turn on and potter about and once at level required,just dive in,no messing
Always good to fit independent non return valves if you can,customer will pay if explained need ,even if fitted in valve have had many fail but as you said many valves now ,for some reason do not have them,china springs to mind for some reason