I'm not sure if it's solely because of the lanterns or if it's because I just fought an Ice and Fire dragon and the forest caught on fire, but I've already had to close minecraft twice because of a lag (the one where entities and items don't respond at all, forgot what it's called) the first time I encountered these lanterns in a survival scenario.
Checking the logs, I found this:
[15:32:09] [Server thread/WARN] [minecraft/WorldServer]: Keeping entity mowziesmobs:lantern that already exists with UUID 618607cb-5e6c-493f-860a-c62a9d030a6a
And I'm not sure if they have something to do with the lag, but these also appeared:
This one came up as I loaded the world in the forest:
[15:32:06] [Server thread/WARN] [minecraft/EntityList]: Skipping Entity with id minecraft:
And this one when I flew around the forest (with wings from the Wings mod):
[15:32:12] [Client thread/WARN] [minecraft/NetHandlerPlayClient]: Received passengers for unknown entity
Again, not too sure if the lanterns caused those since I have a pretty good amount of other mods installed when these occurred.