Artillery, including AAA is a science. In reality, AAA were fired in batteries, which were linked to radar or optical rangefinders and predictors, and which had well drilled crews feeding them. Time-to-target would vary for every shot, and current automatic fuse setters are quite quick. Automatic fuse setting would be simply part of the process. 20 RPM isn't unreasonable for an initial rate, although rate of fire would be limited by barrel heating fairly quickly. However, unless you want to get into battery survey and co-ordination, there's going to be some game-y aspect to their inclusion. However, seeing a Spitfire zoom down to machinegun a flight of Lancasters isn't all that realistic either. Can be fun though...
Enjoy the game, adapt to its circumstances.