That's not true for Premiere Pro CS5 at least on a MacPro (or PC) equipped with one of NVIDIAS CUDA compatible grafikcards. I do own both: a MacPro with NVIDIA's Geforce GTX 285 and Adobe Premiere Pro CS5. Actually, even applying color correction filters on native AVCHD clips works well without rendering the clip. In fact the gpu does all the work in the backround on the fly....I have read more than a few posts about people not liking to edit AVCHD natively in Premiere Pro and Sony Vegas Pro 9....it seems machine performance suffers greatly.
P.S.: I switched back to Premiere after 10 years of experience with FCP and don't regret it yet.