Hard drive: no errors, but slow

Why would it be that a hard drive has a clean SMART report and passes extended tests without error yet is still slow, and what might be done about it?

Thanks.

Have you checked if the disk is fragmented?

https://www.sevenforums.com/tutorials/11733-disk-defragmenter-open-use.html

Yes, it's defragmented, and even optimized.

Thanks.

A little more info. When I first tested the disk on HDTune, it had a terrible graph. It spend a lot of time in the very low transfer speed area. But after chkdsk and the extended SMART test, it got a lot better. There are only a couple of areas where speeds look low. Average access times aren't great, at around 23ms. Average transfer speeds were around the mid-40's MB/s, IIRC. But I did a manual copy test of 1GB, and it came out closer to 21MB/s. The drive on my desktop, also SATA II, is thrice that speed.