Server overloaded can't keep up

Had a similar issue in 14w34b in Singleplayer. I opened an existing world I had just started in 14w33c❓ and I started in a completely different location from where I had exited. (Saved flying near cloud level, started in a 3-block deep river.) This world was created with the Isle Land customized preset, and I had already explored a good-sized area.

FPS fluctuated from 1-3, with occasional spikes to 7.
Chunk Updates usually stayed 0-1, it took about 5 minutes for the screen to load once I entered the game.
Game music was not playing at this time, so I don't believe it was involved in the lag as it usually is.

After the screen mostly loaded, and FPS "rocketed" to 23-27 with a steady Chunk Update of 0, I attempted to jump. Each time I hit the spacebar, FPD dropped to 0-1 and Chunk Update fluctuated between 0-3. I slowly cycled between the Main Menu ("Pause") and the game as I usually have to do to get the world to load and get halfway decent FPS, but this time it didn't seem to help.

Finally, I was able to get the FPS to remain somewhat stable in the 13-17 FPS range, and I jumped and was able to enter Fly Mode. As soon as I flew up about seven blocks, landscape started to load, so I entered the Main Menu again and watched FPS jump to 27-35, and Chunk Updates jumped to 60-75.

Throughout the whole problem, I had the F3 screen up, and my memory use stayed below 60%, running through the standard launcher.

I've attached the console report, and it only mentions a few drops in performance compared to what I had during the exploration of the world in the previous snapshot. The "Can't Keep Up" message was much worse in 14w33c, but it seemed much faster.

Console log showing roughly 5 minute loading of the game.