I experienced the same and it turns out to be a few areas of issues. One will drive you batty.

The dlink and others are often new, in the box with outdated firmware. As such, they need an update first before they work well. Same for client devices.

The second item was that surrounding clients were at 11mbps speed and the WAP was running at that old speed for the benefit of the other clients. New firmware seems to have cured that install.

The last issue is ADHOC mode. If you did that, then it runs at 11.

Bob