this happens in 1.15.2, restarting game fixes it.
i can confirm: this is completely broken. Mac server, windows 10 client, both connected to dumb 100BASE-TX hub. mac firewall is ON. windows defender firewall is OFF, network type is 'private'. PC can connect to mac OK with 'Direct Connect' host:port, but game does NOT appear in 'Scanning' list. both Minecraft 1.12.2
I can confirm this with 1.11.2.
it even happens if you widen the portals on both sides, or if you change the orientation of one/both of the portals.
this is an EXTREMELY annoying bug since good horses are so hard to find/breed and there's seemingly no way to prevent it happening.
please, at the very least, ensure that the hose is oriented so its head is sticking OUT of the portal, and not into the portal block.
just want to clarify that this bug happens at any render distance setting. you just have to be at the correct distance from the flowing water for it to trigger.
Here's a map that repros this in 1.7.4. stand in the pink wool square and set your render distance to 2.
I'm getting the same thing on 1.7.4. it oscillates between 110fps and 2fps. i have a bunch of water dispensers on a clock nearby.
it has something to do with render distance. its seems as if the bug triggers when the dispensers are near the render distance boundary - when i walk away and they've just disappeared into the fog. it doesn't matter if the render distance is 2 or 16. if i'm close enough the bug doesn't happen. if i'm far enough away the bug doesn't happen. but if i'm just at the boundary then the fps drops like crazy when the water flows.
i did use MCedit to remove everything i could from the world in order to find out what it was that's causing it - it's a combination of the mob trap and the strip mine. remove either of those and the problem goes away.
however, the issue also occurs in the un-modified version of the world.
this is because the mincraft.net server is incorrectly parsing the browser header. my browser is sending
and the server is incorrectly interpreting that as Spanish, when it should be responding with English.