We're running a game application .WAR on Tomcat 6 on an Amazon EC2 server, 8 core processor, 7gb RAM. The application uses a MySQL database hosted on Amazon RDS.

This Facebook application takes ages to access when a mere 20-30 users are playing it. Big difference from 1-2 users. The entire .WAR is ~4mb, all static content hosted elsewhere.

The server has never been close to running out of RAM. The CPU utilization has never been higher than 13.5-14%. Even with ~500 users that completely slowed everything to a standstill. The thread count or threadpools isn't close to being maxed out. I heightened maxthreads but it didn't make a noticeable difference.

My theory is that Tomcat can only use one processor core, which would explain why it was slowed to a halt even though CPU usage was stably at 13-14% at the activity spike.
But I'm struggling to understand why it would only use one CPU core. There is no processor cap in server.xml. The app contains several servlets (4 or 5). There is no mention of SingleThreadModel in the Java code.

WHAT could be causing the application to run extremely slowly? If there is only 1-5 people on the application it runs fine. With 20-30 people it's barely contactable.