Kinda a hard question and I dunno how you would test it. The Vista bootloader doesn't work with the XP one and vice versa (i think). I'ld *guess* the Vista *bootloader only* is faster. The Windows xp bootloader opens autoexec.bat which is now redundant and isn't included in Windows Vista (and later) and being an older bootloader isn't as optimised as the Windows Vista one.
This is only my opinion... hopefully someone who knows more than I do will share their 2 cents (or 2p if your from the UK like me