Poor performance in Adobe typically comes from a combination of areas – Low RAM amount, underpowered GPU, storage type/speed.
While the official Adobe requirements list 4GB (and 8GB) as recommended), I really think it should say 16GB minimum. Systems that I run with 32GB or more of RAM don’t have any problems. Your GPU falls in the same area. Most iMac computer GPU’s are just modified mobile graphics cards…so they aren’t terribly powerful. Unfortunately, on an iMac you’re basically stuck with that.
Also, the type of storage you have can dramatically impact playback performance. One disk VS a raid setup and how that is connected – USB, FireWire, Ethernet, etc. Plus you need to factor in what flavor of codec are you using and is your system able to cope with that – ProRes, DNxHD, DNG, h.264, etc.
Ryan Holmes
http://www.ryanholmes.me
@CutColorPost