The A320 flys quicker than the Boeing 737.
In the cruise the A320 flys at around Mach .74 to M.76. The 737 at about M.72 to M.74.
The A340 flys slower than the Boeing 747.
The 747 bashes along at M.84 to M.86 and even faster if needed. The A340, from observation, as it is the only one in the list I have not flown, at around M.82.
As to why, Thats what they were designed to do... As to why they were designed to do those speeds. No idea.
L337