How fast should my MBP internal drive be?
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.