Why gimp 2.8 slow on ubuntu 12.04?

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http://askubuntu.com – I am running ubuntu 12.04 64 bit on 4-core cpu and what i am facing is that when i open a .psd file or a heavy .xcf/.psd file my gimp sends me a "force quit" message. I am using gimp for professional image editing so please tell me what i should do. Thanks screenshot1-usage 2.2gb of memory by gimp screenshot2-the time i closed gimp-4th core 90% and drops to 15% (HowTos)