When close to and looking at a large field of Bamboo, frame time spikes to 300ms or more occur regularly (with frame time dependant on amount of Bamboo), lined up with memory garbage collection (the memory usage goes down corresponding to the spikes). This is even though average frame rate is still close to expected.

Not looking at the bamboo, or being 20-30+ blocks away causes frame time spikes to disappear. This was discovered with growing bamboo, fully grown bamboo does not cause these spikes. Testing with higher random tick speed (and thus higher bamboo growth speed) increases size and frequency of spikes, so the spikes are caused by bamboo growth. Spikes are also observed when harvesting bamboo from a large field of bamboo, or placing or destroying any block in close proximity to a lot of bamboo.

Conjecture: Long frame times are caused by lighting updates and block updates associated with growing large amounts of bamboo in fields, and harvesting bamboo from large fields of bamboo.

This was tested both on a multiplayer server and on a superflat single player world.

Videos (YouTube) of 50x50 field of Bamboo:

Spikes at normal random tick speed, and showing effects of proximity and facing direction

Testing 1000 and 100 random tick speed, fully grown bamboo, and harvesting