"What would a plumber be able to do?"

A plumber would examine what you described at your kitchen sink, diagnose the problem, and fix it.

"The hot water heater is old, but not sure if it needs replacement."

Sooner or later you will go to the bathroom and turn on the hot there (bathroom faucet) and report whether it also has little pressure. Or try the shower or tub.

This allows you to use a process of elimination. If hot water is fine at the other places, then the problem is more likely to be the kitchen faucet, or in the pipe. Stay tuned for more. Post again.