Issue with 1.15.2 Server

My server is just vanilla 1.15.2 no plugins or anything. It started lagging after not doing anything that could cause lag. The lag went away and my friend can play fine but whenever i try to join i just get kicked after about 15 seconds, not being able to do anything. I tried to restart the server, restart the game, and switching accounts. I don't know what else to try so if anyone can help let me know. Thank you.