Speed was probably quite important for night bombers!
1. Time spent crossing the ground-radar covered "night fighter zone" - you don't want to be lumbering slowly across this corridor, which stretched N-S across Western Europe.
2. Comparitive speed - the Me 110 and Ju 88 weren't anything like 100-200mph faster than their prey once you consider the effect of weight from added equipment, and extra drag from the antennae, drop tanks and exhaust shrouds, etc.
3. Endurance - the nightfighters like Ju 88 and Me 110 had barely adequate endurance for the role - if they had to chase a bomber down, they could easily run out of fuel and have to RTB.
40mph could quite possibly be the difference between life and death for night bombers.
PS: Wasn't that de-skinned Wimpy due to a direct flak hit, not nightfighters?