In days of old before I got bold and moved into an intruder.. We had a sticks and blocks.
Well the hot water was running slower and slower and slower.. We replaced one of the water lines to the water heater with copper and it helped but it still kept getting slower to the point where you could pass water faster than it did.. but like and Rv with the standard flow restricting water pressure regulators. the initial surge was good.
finally I did a bit of thinking, narrowed the "suspect list' down to one specific run of pipe and applied hacksaw and wrench.. yup, serious Arterial Sclerosis (clogging of the arteries).. Took old 1/2 inch pipe to plumbing counter at hardware and said "Duplicate in 3/4 inch" they did.
wife went to do the dishes that evening (She was out when I did the repair) and turned on the hot, it flowed well, just like normal.. Only it did NOT slow down, kept on flowing well.. I can tell you my shower that afternoon was long, hot and lovely. (As was hers later in the evening) We worked crossed shifts.