I use the Panasonic HDC-TM90 camcorder and it generates AVCHD (mts) files... But when I try to play them
on kdenlive, it takes about 5 to 10 seconds or more to start playing... I have a Core i7 930, 6GB RAM, 1.5TB HD...
so I'm sure it is not a hardware problem.

Is there a reason for the playback to be so slow? Thanks.

There is no hardware acceleration in kdenlive, proxies are available to help.

But the problem is not with the playback. The problem is with it taking too long to start. After it starts, it will
playback fine...

I was unsure whether that was what you meant or not, are you using svn build? I have after your post noticed same or similar problems with delay in playback starting for my sources which are h264 and mjpeg in recent svn builds.

.mts playback

I have a sony hdr-cx130 which also saves the hd video files to .mts
and on my computer a intel q 9550 with 2 gig of memory the .mts files wont play back properly untill I use kdenlive to render them to .mpg4
then thay play back fine using xine and kaffeine for playback.
I dont know if this will help you but give it a try!