The 20mm anti air guns nations currently have are simply too weak to efficently take down planes, ESPECIALLY planes that are br3+ and ai bombers.
Just like anti tank guns get stronger the higher br you play, so should be the case with AA guns.
Would love to see 37mm german, 40mm british and so on.
Not sure if slower fireing AA guns would improve the situation in top tier were all the jets are hard to hit and windows of opportunity of actually hitting them beeing very short before it has passed.
Having your AA gun lined up to intecept the jet only to miss the shot because the AA gun failed to shoot when it mattered due to the low rpm is certainly an opportunity.
37mm AA guns would only be better if the plane fails to take evasive action but at that point even the 20mm can lite it up easiely.
The damage of the 20mm AA already is preety good most of the time.