How fast should my MBP internal drive be? I'be been experiencing an ongoing (if not precisely measurable) feeling that my MBP is underperforming, and it seems to be related to disk IO. I recently downloaded a disk benchmarking tool, to see how the disk is performing. The issue, however, is that I don't know from that how the disk should be performing. Where can I find numbers to set my expectations?

This is a late-2010 17" MBP with a 7200RPM Hitachi HTS725050A9A362 drive.

I'm also not sure 100% if the issue is in disk I/O; I'm trying to narrow the problem down. Other slowdowns, for example, include text entry in this form; typing the word "slowdowns" back there, for example -- I'd completely finished typing it before the text appeared in the form field. So, it seems likely that there are other things going on.
Yep. That's not a disk issue. My suggestions / theories:

FileVault could be a reason for slowdown, and a pretty big one if your disk is very cluttered. Even with FileVault off, disk clutter can slow down your computer if it's excessive, specially on startup (though not when you're typing). I consider "disk clutter" to be about 80-90% of your disk space used. 

RAM could be a factor too, especially if you're running memory intensive apps. Your manual should have some more info on how much RAM your MacBook can have: up to what you can expand it (perhaps 8 GB). When your RAM gets full (you can see how full it is in Activity Monitor), it starts transferring memory pages from RAM to your HDD, which is considerably slower than the lightning fast RAM. Perhaps this could be an issue, because when your apps need that cache, it needs to load it from a slower source.

And last, but probably the biggest: your apps. Check if any apps are eating your CPU in the background using Activity Monitor (Applications > Utilities > Activity Monitor). Also, check on System Preferences for any apps loading when you login (Accounts > Login Items tab). Try to get rid of some background apps you might not need. Flash is a CPU hog, and it has happened to me that it slows my whole web browser down to the point where even typing is slow.