Write speed is always slower than read speed plus as the drives heads work their way into the center of the platters the performance drops due to the slower spinning of the platters. How full is your drive? when was the last time you ran defrag?
Going from ide to ahci on an hdd is not a huge increase in performance. A mechanical drive might use a high speed interface but it cannot use all of that bandwidth, not even half of it.
Very true but I doubt it would deteriorate right down to 2MBPS. It actually has 2 partitions, i am referring to the C partion which is only about 25% full in any case 50gb of 200gb. Its fully defragged. D partition is about 75% full at this time and not defragged too well, 200gb of 270gb approx. I will continue to clear more items and defrag d although I doubt this is the problem and how this would affect performance on the C partition.