The h264 Code is very great and have a high compression.
You PowerBook or MBP nead CPU to decompression the files.
If many layer activatet, need many more CPU speed.
If you have checke a less compression, i like the AnimationCode
(this play great forward and backward, but nead a fast Harddisk)
Find the balance: Harddiskspeed and CPU to optimize the System.
I hope this can help you.
(or have you update your System in the last Days and
the Grafik update!!! If you say slow now)