I don’t know that it’s the fastest growing, but I know that the frum community of Las Vegas is growing rapidly (as was the city itself until very recently).
Check out the Disjointed Communities of Creedmoor, which according to welfare and section 8 records boast 150 quintillion neshomois and about 89 trillion dwelling units.
Lav davka, sometimes places get a lot of people move in or out of to. Most frum places I’m guessing all have similar birth rates (I’m probably wrong, but still…), so it’s a question of moving.
Look up the Disjointed Communities of Creedmoor – Creedmoor needs no explanation!
(The Admou”r meCreedmoor is a self-committed psychiatric patient who claims he suffers from multiple personality disorder so as to be able to register phantom identities to receive Federal entitlement programs).