I have the same ST3320620AS drive and had poor performance with it since I got it back in 07. Your drive looks slower than mine did though. You may have other problems. In my case read speed was much lower then write write speed. About a year after I got the drive I found out that many people had the same problem and it was firmware related.
After much searching I found the updated firmware and flashed my drive. The drives read performance increased substantially. The performance problem seems to only affect drives that shipped with firmware version AAK and the firmware update only applies to drives with the part number ending in 308. The PN and firmware version is printed on the drive BTW. The updated firmware is AAM. The file I found was named ST3320620AS3.AAM.zip. Links related to the problem
A comparision of AAK and AAM firmware on my 7200.10.
Keep in mind flashing your drive is not risk free. You could brick your drive and loose all your data. If done correctly the chances are very low.
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