After exploring a very large section of the map and creating around 77,000 chunks, if I move back to pre-rendered chunks a short distance away the framerate drops down to 0 FPS and will move one frame only every few seconds. Closing the window and reopenning fixes this problem, but it will persist continuously after moving a distance. This is very frustrating when you have distant mines and minecart systems.
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Creating 77,000 chunks is not a reliable way to reproduce this bug. Care to see if it's biome related, distance related, or mob count related?
In some LP videos, they would experience temporary severe lag issues while they are just building/mining stuffs. Usually, when it lags, they are near spawners. I may not be correct, but it's a good direction to look into seeing if spawning mobs and loading chunks at the same time may be causing the issue.

Please post more information in regards to your system configuration, such as graphics capabilities, RAM (and allocated to game), java version, etc.
Resolving all FPS/Lag issues to MC-2176.
LOADS of people are experiencing this but no one seems to care. I totally agree that this is frustrating cause I have this problem too, especially in a developed area.