Limited pressure in bathtub

We are having limited water pressure in our bathtub. All other faucets in the house seem to be running fine. We have a Delta Monitor single handle faucet and replaced the cartridge assembly, still no difference. Any ideas on what the issue could be?

faucet

Possibly a bad installation with undersized piping.

The cold water seems to be running fine as does the water to the bathroom sinks. We checked the pipe size and it is correct. Any other ideas?