I used the Spark mod profiler to identify why our world of only 4 people was lagging so badly. I expected it to be Minecolonies or Create or one of the mods we are deeply involved with. Turns out it's just the whales!

Here are two profiles from two different points in time:

Lagging VERY badly: https://spark.lucko.me/735awjXV8Z?hl=88,63
Lagging just a little: https://spark.lucko.me/LXhL8TKk7J?hl=77

During the lag using /forge tps minecraft:overworld reports around 7 TPS. I used a command to kill all the whales and it went straight to 20 TPS, but it does return.

I would estimate the lag happens for about 2 minutes, and occurs once every 30-60 minutes. This only became a problem when a player started building their base near an ocean biome so whales are frequently spawning around there. I haven't identified what actually causes the lag but it's happening naturally. Nobody was messing with them in any way.

Is there a way to disable whales from spawning?

EDIT: I just found config/alexsmobs.toml has a line "cachalotWhaleSpawnWeight = 2". I set that to 0 and hopefully that will fix the problem for us. I'll leave the task open so this can get looked at.