https://youtu.be/EVRVXTphm1Y
This is an issue that has plagued the game since 1.13.1 and is still present until 1.15.1 pre1
This has been a serious issue since and has not been properly brought to light because majority of people run on modded versions of minecraft like Spigot, Paper etc that optimises the server by disabling majority of the game's features/mechanics.
For vanilla servers, this is a huge contribution to the lag, because fishes would constantly spawn and despawn in masses of hundreds. This causes the overall mspt to spike in high numbers which usually causes the TPS to drop, unlike the video where it stayed at a cap of 450~, the usual is a lot higher example 700~ that commonly occurs. Server mspt in that mass usually goes over the average of 100~ mspt,
Furthermore in the video the mspt can visibly been seen due to the large amount of Fish AI in process. Due to such this makes it visibly impossible for a vanilla server to continue without any external means of modding or a genuine vanilla experience.
To Reproduce,
1. Install any tracker example, Fabric Carpet.
2. Load a fresh world.
3. use /log tps and /log mobcaps to track the spawning and server tps
4. Turn of all mod spawning using /spawn rates [monster/misc/water_creature/ambient/creature] 0
5. /kill @e[type=!player] then /kill @e[type=item]
6. /spawn rates water_creature 1 and idle over a large ocean
7. Results should vary of fishes spawning over hundreds from the mobcap and mspt/server resources absurdly spikes over possibly causing server TPS to decrease