When the terms performance and slow are used they can be difficult to quantify. In my testing of this I would notice a delay of 2 to sometimes 3 seconds before the preview would load but this is on a new Intel based iMac. How many seconds is it taking for the preview to display for you? I know for sure the amount of ram you have is sufficient but I'm not sure if the processor is the cause of the problem you are experiencing. Please post back with this info and if there seems to be something going wrong that needs to be looked into further I can open a ticket for you.