The hot water in my house used to be provided by an immersion heater fed from a cold water tank in the loft (2 decades ago) and was distributed by 22mm pipe. When a gas boiler was installed the original pipes were left in place.
Having recently had the boiler replaced (with a higher maximum hot water output) I investigated what I could do to reduce the time taken to get hot water out of the tap and discovered that the 22mm pipes leading out of the cupboard went further than expected.
A 22mm pipe holds more than twice as much water as 15mm pipe so this results in a lot of unnecessary water in the pipes between the boiler and the taps. While this has little impact on the shower (mostly fed by newer 15mm pipe), it doubles the time taken for hot water to reach the tap in the sink because almost the entire run was in the old 22mm pipe (7 metres of it).
Even at a fast flow rate of 6 litres per minute that's an unnecessary additional 13 seconds for the hot water leaving the boiler to come out of the tap in the shower room and an extra 8 seconds for the kitchen sink at 10 litres per minute.
All of the 22mm pipes have now been replaced with 15mm pipes and the hot water now reaches the tap faster: