Sylvan, to be technically correct, lower speeds are recommended to limit erosion. Any water speed will create erosion. Gases and minerals in the water will create corrosion. High speeds will create unacceptable erosion and corrosion. 2 FPS or less is recommended for water over 180F, as turbulence creates cavitation as the water changes to steam in the pressure drop after the obstacle and then collapses after the local low pressure zone.
For ordinary domestic water temperatures below 140F the fps can be higher, but no higher than 5-7 fps; at those speeds, the protection of laminar flow is eliminated even in the straight sections of tubing – producing noise and increased wear from the friction of entrained solids and exposure to gaseous corrosion.
2 fps is exceeded in a 1/2-inch shower stem as a shower head can deliver 2.25 gpm and 2 fps in a 1/2-inch pipe delivers 1.89 gpm, so the 1/2-inch pipe will exceed 2 fps in ordinary shower piping.