Everything was running smoothly until I got to Sentinel Island, at which point I noticed that the sound would often stutter for a second, but other than that it was still okay.
But after reaching a certain point in the Battle Tower, I decided to go back to Broken Valley and finish up everything there. Immediately upon entering BV I noticed an extreme performance reduction in the graphics -- very stuttering and jerky, which I suppose is a framerate reduction. But I have no idea why. It's the same areas I had been to before without any problems.
Are there any known issues that occur after you've played a certain amount of time, perhaps? Shutting down and restarting didn't help and nothing else was running in the background. It was just a sudden change in performance.