It appears you fellas were correct about the whole MTU thing!
I had initially left the MSS clamping text box blank, assuming that the default 1400 would do the trick. However, after some fooling around, I changed the MSS clamping setting to 1300, and the speed shot up! I then changed it to 1400, and the speeds once again slowed way down. Once again, I set them to 1300, and the speeds shot up again, so I think it's safe to assume that was the issue.
Now I'm curious as to why this was the case between these two boxes. Can anyone give me a high-level explanation, or perhaps knows of some documentation that would explain this issue?