engine-eer
The reason that these aircraft have slow initial acceleration is that they have turbojets. Turbojets have higher velocity, lower mass flow rate exhausts, and at low speeds the propulsive efficiency is poor. Think of it this way, bigger fan, better low speed thrust, prop, even bigger low speed thrust.
That I understand actually, it's more efficient to accelerate a large volume of air a small amount, than accelerate a small volume of air a large amount (though the faster you go, you need a higher exhaust speed). For this reason propeller-driven fighters often had more thrust/weight than early jet fighters, though as the speed increased, many of the jets had them beat (ram-compression was another variable).
The early turbojets took forever to accelerate.
Yes, but it wasn't due entirely to a fast-exhaust velocity; some of them were quite underpowered: The P-59 had a T/W of 0.32/1.0 to 0.36/1.0; the FH-1 at 0.27/1.0 to 0.32/1.0; the F2H at 0.32/1.0 to 0.36/1.0 (which are comparable to modern commercial airliners).
Those really long runways for the B47, B52 and the early fighters were necessary because the early turbojets didn't produce much low speed thrust.
As I understand the B-47's weighed around 200,000 and had 6 x J-47's which produced around 6,500 to 7,500 pounds of thrust, IIRC, and with that being said, that gives you a T/W of 0.20/1.0 to 0.23/1.0; The B-52's weighed around 420,000 pounds fully loaded with 8 x J-57's producing around 11,000 to 13,500 pounds of thrust yielding a T/W ratio between 0.21 to 0.27
Regardless, the point I was making was that two aircraft with the same T/W ratio, one with a subsonic inlet (short, round-lipped, bell-mouthed), the other with a supersonic inlet (long, sharp-lipped, covergent-divergent); the one with the longer inlet tends to accelerate more poorly.