I remember seeing a source explaining that ideally Megilla should be read somewhat fast because it is called "iggeret" (a letter), and letters are read at fast speed out loud. I've also heard the viewpoint supporting slow reading to facilitate listeners' following and understanding.
Do any halachik sources discuss speed limit for reading Megillas Ester? For example, if the reader is able to pronounce words clearly at a very fast rate of speed (say 12 minutes for the entire megilla), would the speed that's too fast for most people to follow invalidate the Megilla reading?