I wonder if EFI boot with something like efistub would be a even faster. Afaik EC2 BIOS boots which has some fixed overhead (although Clear Linux still manages to boot very quickly)
Also on boot, I /think/ EC2 pulls down the AMI to EBS from S3 so theoretically smaller AMIs might be faster but not sure
My experience with desktop hardware is that traditional BIOS boots faster than EFI, because the former does much less as well. (This is not the difference between "BIOS boot" and "EFI boot" options in an EFI BIOS, but rather a "pure native" BIOS vs "pure native" EFI with the same motherboard.)
Was that with or without a bootloader? My understanding was EFI was marginally faster to initialize (which might not be true based on what you're saying) but you could also skip the bootloader and load the kernel directly (EFISTUB) which would also eliminate initram (which seems fine assuming you're using vanilla VMs on popular hypervisors/providers)
I've tested both EFI and BIOS boots on EC2, and they have about the same overhead.
And I've tested AMIs that are no bigger than a Linux kernel and a single static binary; times are still on par with what's listed here. Still takes 7-8 seconds before the first user-controlled instruction gets to run.
Also on boot, I /think/ EC2 pulls down the AMI to EBS from S3 so theoretically smaller AMIs might be faster but not sure