Actually its not, changing a magzine slows you down. In the case of a bolt action tht is sort of, hmm, but it does slow you down.Same overall speed but now your rifle is better balanced and the magazine isn't as exposed.
The Brits were into rapid fire and a well trained Infantryman prior to WWII could shoot a SMLE in the low end area of an M1(realistically no - factually as your well trained Infantryman was taken out of action and replaced by a conscription that ability went by the wayside)
Still, the SMLE put out an amazing amount of fire for a bolt action. Slickest bolt of any gun I ever handled.
But it correct that there is a balance. Lots of stores out of Nam about being out gunned. But that 30 round mag on an AK-47 also meant you had to be raised up higher to shoot (along with the rim issue making it longer still).
The Bren was a good answer to the keep it low issue. The BAR would have benefited with that setup greatly and with a larger magazine.
A conscript army gets round envy. WWII infantry would have laughed (they went up against MG-42 with M1, but everyone had an M1, fire was spread out, a Germansquad with rifles was there to support the MG-42 gunner carrying extra ammo in belts for it and side support to avoid a flanking).
Now, the troops are trained to fire semi auto unless there is a reason for full auto, save ammo. The Mk-27 is an example of a change in though process back to a heavier barrel with sustained longer range accuracy and spread the fire around a squad better.