My water heater was replaced last month. Since then the hot water in the bathroom sink has been very low flowing. I've disconnected the tube leading from the hot valve to the wall and checked the water flow. Its very low flowing. The hot water throughout the rest of the house including the shower have good flow to them. The house was built in 1943 and I have never tinkered with the plumbing.

Any suggestions short of replacing the pipes?

There is something trapped in the hot water stop valve. Probably solder from the water heater replacement.

Turn off hot water at water heater. Remove bonnet nut and then remove stem of hot water stop valve under the sink. Place pan or bucket under valve. While holding a cloth over the open valve have someone turn on the water at the water heater briefly. Very briefly. That should flush out stop valve. Replace the stem and bonnet nut.

Or you can just replace the stop valve.