I have a hosted MC server at g-portal (4GB RAM, 12 slots) but compared to 1.12.x the server is extremely laggy and using a huge amount of RAM. I don't have much more details of what hardware that server has as it's a hosted, but i've used the same servers for quite a lot 1.12.x modpacks without any lag problems or using as much RAM as the 1.13 vanilla server now so the hardware should be plenty enough to run a vanilla server with no problems too.
It's impossible to move by boat or fly around in creative / spectator without flooding the servers log with "... is moving too fast" / "can't keep up" errors. (Resulting in a lot of rubberbanding for players) This is ONLY happening in ocean biomes. (We're in a seed with a ~700x700 ocean around a small island) Once i'm in areas with no oceans and just flying around to generate chunks over normal land this never happened so far.
RAM goes up from ~400 initially used to over 4GB within 10 minutes while exploring with only one player on the server.
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If it was a CPU issue the CPU load should be way higher then just the 15-20% spikes i see while this happens. Also it doesn't explain why this only happens in ocean biomes but not on land.
The RAM issue is just another issue beside the lag spikes: 4GB RAM used after 10 minutes of exploring looks like a serious memory leak to me. This is nowhere near of what a <1.13 vanilla server with only a couple people ever used.
(And imho if the CPU is enough to handle 100+ mods in 1.12 with hundreds of machines running at the same time, it should easily be able to handle chunk generation in vanilla)
Looking through some other issues my guess would be that it could be related to MC-135289
When water updates take like 10 times longer as in 1.12 and (as far as i understand how it works) it generates a lot of water/ocean chunks generating these chunks would cause like 10 times more lag then in 1.12
Java will use as much RAM as i let it with xmx, but Minecraft doesn't fill all that reserved RAM... Press F3 ingame with xmx 4GB and it will report something like "Mem: 14% 609/4096MB", where the first value goes up and down all the time (clientside)... But on the dedicated server this value doesn't go up/down (viewable via RCON), it goes up and up and up until it hits the 4GB xmx while exploring ocean biomes.
Classical memory leak
Is this MC-91621?
It doesn't sound like it could be that. Neither was i flying >125 height nor are boats anywhere near that height.
Moved too fast and cant keep up messages suggest that the server has a slow CPU, not a RAM issue, have you tried with another provider?