Not the fastest in all of T20 history, no, but it's a huge deal. It is the fastest in T20 World Cup history. Globally, it’s tied for the joint third-fastest T20I century ever. The actual world record is held by Sahil Chauhan from Estonia, who did it in 27 balls. There’s also Muhammad Fahad with a 29-ball one. But Allen’s knock is now the fastest T20I century ever against a Full Member nation, beating Rohit Sharma and David Miller's 35-ball records. So, in big team cricket, he’s basically the new king.
