These programs use a ton of hardware resources. Most detailed answer you could get would be to go to the Adobe Hardware Forum (linked through the sidebars on the Overview page) and that forum's Tweaker's Page, download and run the Benchmark testing ... and the results will tell you exactly where your system works ok and where it ... could use some work.
The answer to your question could be say that you need a better lay-out of programs across multiple discs because of read-write conflicts slowing things down, resources competing for CPU/GPU attention, all sorts of things. Or maybe your system is running ok but the effects are more than it can handle real-time. Or you need a re-install or something.
Thanks Neil. I've since worked out it's only happening with Canon .MOV/h.264 files, my Panasonic GH2 clips play smoothly with effects on them. Does anyone have any idea why there would be a difference?
So it's Canon MOV/H.264 files. What are the Panny ones you're shooting? On my GH3 I mostly shoot the MOV files rather than the MP4, and avoid the AVCHD which can have more clumpy noise in them according to both my own experience and that of say many of those who actually test their cameras thoroughly over on the Panny section of the personal view forums. Yet some people love their AVCHD off their GH3. To each their own?
And running that benchmark test will probably tell you all sorts of interesting things, as it's designed to rag out any computer in real-time work with a variety of effects & "tools" that push every part of the system, disk reads & writes, CPU, memory, cache, GPU, vRAM, everything. Also, it was noted in the hardware forum recently, it may very well be worthwhile to go through your machine's resources info, noting which IRQ such things as disc busses, GPU, and such are assigned to. One of the people found that although his system had told him there were no conflicts, the GPU was routed through the same IRQ that a couple other "heavy" things went through. Just figuring that out, then going back to the BIOS level to re-direct where things got hooked up through made a system that was frustrating him as it ran WAY below what it should ... now stand up and get to work. I think I need to do both the benchmark and the IRQ study on mine. Sigh.