Lag when online.mode=true.

I'm having problems with lag when I open a vanilla server with online.mode = true. When I set it to false, it's normal.

I just created the server to test, with no maps already created.

I made a video with each setting to see. I haven't tested it on other versions, only on 1.15.2 and 20w14a

I was using my local IP (without anyone)