I don't do HFT, but with what our company does, I've spoken to a few people who do. So take all of this with a pinch of salt, but as I understand it, your execution and autonomous trading systems live in the same rack as your exchange/dark-pool endpoints. Your slower strategies can be offsite, as is the Big Red Button to close down your positions. Still, faster's invariably going to be better, all else being equal...