Partitioning a drive wont help (as I have been informed in the past.)
I'm not looking for someone just to say buy a new computer or hard drive- that is not helpful as I have had no problems with AVCHD in the past on the same computer with the same software so i'm looking for reasons to improve playback other than chaging resolution
Then use different source material that is easier to edit. This is woefully inadequate for the task you throw at it.
You expect your granny to climb up a steep hill with 150 pounds in her backpack? Give her a 15 pound backpack and it still is a serious uphill fight.
I'm currently editing with bigger AVCHD files in another project which play fine at full resolution.
I've just converted the movs into proxies, 640x480 jpeg2000 and made a sequence from clip settings in pro and it still plays stuttereing playing around 1 frame every 5 frames, in quicktime it plays fine, this is an exmaple of the file I'm using...