Game FPS drops when near entities

I spawned a ton of pigmen and walked near them
Expected result: the game does not have obvious FPS drop (applies 1.5.3 and before)
Actual result: game FPS has a very obvious drop (1.6.0)
I found that this does not only apply to a group of pigmen, but also other entities like players in any numbers. (Even standing next to a single sheep causes lag, unlike pre-1.6)

Device: iPad 6 (not Pro, please add an option for iPad 6)