Upon loading up an extant world or creating a new one, the game mostly runs at 55-60 fps, but will intermittently and completely stop for 0.25 to 1.5 seconds. This does not seem to happen if the player stands still.

With the debug screen open, a single spike visibly causes the frame rate to drop to 27, 14, or even 0.

Pressing Shift + F3 to open the debug menu with graphs sometimes causes the game to crash with an unknown error.

EDIT 2018-10-24 22:24 - I just managed to get the graphs without a crash. Attached is the FPS graph showing one such spike.