Server keeps missing ticks

I started up new server first on 19w08b and after that 19w09a.

Right after creating console gave warning 

[WARNING] Can't keep up! Is the server overloaded? Running 2267ms or 45 ticks behind

New warnings comes endlessly.

RAM peaks critically and doesn't become stable even if nobody moves on server.

I created same world in singleplayer and it kept spiking RAM 
(GPU: i7-870/2.93GHz, RAM: 16Gb, 5Gb to game)