In CS you can leave the buy zone immediately. I don't necessarily believe that Valorant's decision to fence players in their spawn for the first minute during the buy period is simply to save on server costs, especially because they realized that optimization possibility after the fact. Being able to buy your weapons quickly may have an element of skill, but it doesn't make for particularly interesting gameplay. They may have just decided that they'd rather level this part of the playing field so people can focus on the core tactical FPS gameplay.