The hot water pressure is low in my shower and tub
Does you shower have a pressure balancing valve?  That is the single-knob type to control the water temperature.  If so, there is a pressure balancing piston that sometimes gets clogged with hard water deposits and stops working properly.  

Fortunately the inner cartridge can usually be replaced and restore proper operation, without having to replace the whole valve.  See this TOH video for and example:

http://www.thisoldhouse.com/toh/video/0,,20809251,00.html