You cannot really compare AVCHD playback performance of Avid vs Premiere, since Avid transcodes everything into its own highly optimized MXF (or OMF) media files (AVCHD file transcoded to Avid must be really HUGE), whereas Premiere plays the original AVCHD media. I think Adobe should add some intermediate codec too, but that’s another story.
AVCHD and MPEG4 in general is optimized for distribution, and I scratch my head what lead camera manufacturers to use it as an acquisition format.