"... a classic hot water flow issue, ...
Shower valves are usually fed with 1/2" pipes, and with the addition of a shower head, you may get a flow of about 2.5 gallons per minute. ... "
See more about why at this HGTV blog: Ed Del Grande: Ask Ed -- Hot Tub, Cold Shower
I've always run the hot water into the tub first, btw, knowing that it had to get all the way from the hot water heater. I would never wait under the shower head for it to get hot!