Mine is a 2006 ATI MBP (2.33Ghz C2D) with 3Gb memory and QT and kernel_task each were taking roughly 50% CPU to play that video since the upgrade. If kernel_task is taking up 50% I can forget about interactivity!
VLC plays the same video without breaking a sweat so the machine itself is not a problem. So I went ahead, backed up my applications and data, did all the deactivations (iTunes/Audible) and wiped it clean. Next installed plain 10.5 and tried playing the same video - much better 28% QT and 15% kernel_task.
Then I applied the 10.5.7 update from combo installer and it is back to taking 50% percent CPU - although this time the kernel_task has gone down to 35% which might be just be by fluke.
I am now doing the whole backup/wipe/reinstall/re-update drill on my wife's 4,1 MBP which is just used for surfing/email/documents - she is complaining it is unbearably sluggish to boot and switch between apps - was just fine before the update.
Next I have to worry about installing the update on my dad's macbook - he lives thousands of miles away, has a slower 256 Kbps connection and if the update breaks something it is very hard to support.
Only Apple can pull such awesome stunts. It takes competence of different kind to break stuff on minor release of the OS when they control their hardware, software, drivers, applications, and have a tiny user base.