At least if you're using the Ec2 optimized amis, Ec2 instances frequently boot fast enough that they'll be executing your username initialization before you get the run instances response.
Though there's a long tail, so sometimes there can be a gap on the order of a couple second between the sync response and when the bootloader transfers over to the kernel.
Though there's a long tail, so sometimes there can be a gap on the order of a couple second between the sync response and when the bootloader transfers over to the kernel.