I imagine the Spey, designed for the Bucc with a small by-pass, was optimised for low levels. More thrust off the boat and better range at all levels, but less thrust at high level (a trait the F3 inherited) - and also for the drag reasons highlighted above. I guess the J79, like many US engines would have had much quicker response and been better at high level.
I've flown in a couple of US-engined jets, including a German F4 once, and I seem to remember the rpm gauge seemed to be connected directly to the throttle - unlike the laggy response of UK high by pass jets.