1.9 Dedicated Server Crashes When Someone Joins

I load up a .jar Minecraft server, and within a few seconds the average tick goes below 1 millisecond. The moment I try and join the server, the average tick climbs at a steady rate, reach's maybe around 900 if I'm lucking, then crashes. The RAM usage stays under 120, and I thought that might be the problem. So I tried it on a Windows laptop, and the same thing happened. Not sure why this is happening.