At a company I freelance at, they recently upgraded to CS5.5. For some reason when I do a RAM preview it can't playback at normal speed. I've dropped down the resolution to 1/4, and skipping 2 frames. It hits about 9 frames a second no matter what I do. I have a sneaking suspicion it might be the fact there is only 5 gigs of free space on the tower's main hard drive. Any insight would be appreciated.
One thing to check is the output mode on video preview in preferences. Chances are the system has a video card by BlackMagic or Aja. Make sure it's outputting the correct format, NTSC, 720p, 1080i, etc. for your comp. If it's not, AE can get slow trying to make the conversion happen in real time.