I think this may be the same issue as the previously closed MC-75679. Visiting an area with lots of mobs, such as an underwater temple, or a farm with around 100 animals, uses a huge amount of upstream bandwidth, over 120 kilobytes/1 megabit per second. My home ADSL connection is saturated, I start dropping a lot of packets and Minecraft becomes unplayably laggy until I leave the area. It's one thing to avoid player-built environments with lots of mobs, but it's pretty frustrating not to be able to go to underwater temples at all.
Framerate is not affected, and player movement remains smooth, but interactions with the environment and mob movement can be delayed for ten seconds or even more, suddenly all occurring at once. Unrelated programs on the same Internet connection start suffering packet loss at the same time and recover when Minecraft is stopped or the area is left, also consistent with a saturated network connection. The ADSL connection's advertised upstream rate is "up to 1.4 megabits per second".
Reopened MC-75679