Why are the cannons so weak on the 109-E, yet so strong on the Russian planes?

[winny]

RoF is important but not that important..
It's all about how much weight/HE you can get into a target.

To work it out properly you need projectile weight, % of HE in it and RoF then you could see what the difference would be for a 2-3 second burst..

The Russian projectile weighs 95 grams and is 6% HE.
The British projectile weighs 130 grams and is 8% HE
The German projectile weighs 92 grams and is 22% HE

Russian RoF = 13 RPS = 74g HE per second. (shVAK 20mm)
British RoF = 10 RPS = 104g HE per second. (Hispano MKII)
German RoF = 8 RPS = 161g HE per second. (MG-FF with HE rounds)

Then x that by the number of guns...

(this is ignoring the fact that the spitfire had MG's too)
Muzzle velocity isn't that important (although it does have a significant effect on accuracy).