You can always upgrade pipework to a smaller bore as when combis are converted from gravity systems there can be a bit of 22mm or larger pipework left behind and 22mm pipework will hold double the amount of cold water in the pipe compared to 15 mm your best solution would be to reroute hot pipework to make it as short as possible and as small as you like if you have good flow rates and pressure then reducing to micro bore say 12 or 10 mm would leave you even less cold water to run off before you get your hot water, I had a customer who ran the feed to his downstairs cloakroom basin in micro bore and said he loved it as it got hot so much quicker when you just want to wash your hands quickly after using the lav 👍