Ocean biomes lagging server / using too much RAM

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.