There's some lubricant in the track and I assumed that the folks who worked on our bathroom a few years ago had put new lubricant in. Apparently not. I consulted with samwinolj who suggested that the lube looked like white lithium grease, so I picked up a tube of that at Home Depot yesterday. After the kids went to bed, daisy_knotwise and I disassembled the door -- it having popped another screw on Friday. I wiped out the old, rather black lubricant, filled the track with fresh grease, and we put it back together.
This morning, I stepped into the shower and pulled the door closed with the usual amount of force. It slid swiftly along the track.
"Wow!" I said.
I guess it was a bit overdue for being lubed. :)