There was a coin called Flashcoin that tried to do 6-second block times.
I don't think that's being actively mined anymore as another Flashcoin appeared a year later (60-second block times).
Ethereum is planning a 12-second block time. Vitalik makes an argument for why that's probably the minimum for now here. Of course, ethereum hasn't been released yet, but Fastcoin with its 12-second block time is probably considered the lowest block generation time currently.
answered Feb 6 '15 at 19:20
7,1021010 silver badges3131 bronze badges