Slow graphics on external monitor with Ubuntu 12.10 on Macbook Pro 8,3?

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http://askubuntu.com – I have a 17-in Macbook Pro, running Snowleopard and Ubuntu 12.10. Ubuntu runs fine, except when I connect it to and external monitor. It exhibits a considerable lag in graphics acceleration. i.e. when minimizing windows using (super key / command + w); Relevant info Product name by executing sudo dmidecode -s system-product-name is MacBookPro8,3 Graphic card and other hardware info by exectuting lspci gives VGA compatible controller: Advanced Micro Devices [AMD] nee ATI Whistler [AMD Radeon HD 6600M Series] I have searched on forums, exprimented with propritery drivers (which did not wo (HowTos)