Bob-

I use ball valves (usually 3/4") on inlet AND outlet; makes changing
the water heater NEXT time a snap! Just close both valves, drain water
heater, place w/ new unit, turn on valves....you're done. Not water
leaking back from the house to make a mess!

Globe valves can be used to throttle but why in a water installation?

Gate vavles give good flow when fully open but don't have the postive
off like a ball valve.

Don't know about water heaters having flow restriction on purpose,
never seen a situation where the w/h was the restriction;

old, rusty or too small pipe yes; water hear, no

age, condition & material of pipes?

who did the install? could they have introduced something to the
pipes?

cheers
Bob