Your 2011 iMac has only an AMD (formerly ATi) GPU - and you cannot upgrade or even replace it at all. Worse, Premiere Pro CS5.5 does not support OpenCL at all for GPU acceleration in its MPE engine (CS5.5 supports only CUDA, which is used by most recent Nvidia GPUs, for GPU acceleration). Worst of all, your iMac's GPU has only 512MB of dedicated graphics RAM. And because your iMac does not have enough dedicated graphics RAM, Premiere Pro will be permanently locked into the MPE software-only mode.
Thank you very much for the reply.
This is very disheartening. So, there is nothing I can do? Would upgrading to CS6 solve the problem?
Could a thunderbolt external harddrive help?
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>Would upgrading to CS6 solve the problem?
Probably not... CS6 does add support for "some" ATI cards for hardware MPE, but I think the cards still need 1Gig of video ram
But, follow the link inside http://forums.adobe.com/thread/773101 to read more
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Basically, there is nothing that you can do with that 2011 iMac. Upgrading to CS6 still would not have allowed you to enable GPU acceleration: OpenCL GPU acceleration only works if the GPU has 1GB or more dedicated RAM (and officially, CS6 for MacOS only supports Macbook Pros, not iMacs, with the HD 6750M or HD 6770M and 1GB or more dedicated graphics RAM for OpenCL GPU acceleration on AMD GPUs). Your iMac's GPU has only 512MB of dedicated RAM, as I mentioned. That's too little RAM to even allow the enabling of the GPU acceleration in CS6.
As for the addition of the Thunderbolt external hard drive, that would still gain some improvement in performance, especially since your iMac is bottlenecked by the use of only a single disk (the OS disk for absolutely everything). The iMac's internal hard drive uses a SATA interface like Windows PCs use - and a single SATA connection can only pass data in one direction at a time (this is what I meant by "half-duplex"); a block of transfers in one direction (either read or write) must be completed before any transfers start moving in the opposite direction. And most video editing tasks require simultaneous transfers in both directions on a disk interface (which a single SATA connection cannot perform; this is why we often needed two or more disks).
Thanks for that detailed reply. Considering a Thunderbolt harddrive.
A user on another website said "If you plan to edit HD material, your really need a PCIE or USB 3 external raid in order for PrP to function. Media should not be on internal or firewire drives for best performance."
I have never owned one of these. What do you think?
A Thunderbolt drive or raid would be the ideal upgrade to work from. That will be about as good as the performance will be on that IMac with Adobe.