Comment 59 for bug 247960

Adam, you're right in your original post that initrd is the cause of the delay.
Other non-Ubuntu based distros do not have the long 50 secs wait to boot OS at usb. (same kernel)
It's just 13 secs vs 50secs. (haven't tried Debian distros).

One would think after 5 years, something ought to be done about it.