I would first determine if SSRS is indeed only using one processor. This is simple enough using Process Monitor, Task Manager, or Performance Monitor. If it's only using one thread, I would research the documentation and see if there is anyway to scale this out. Unfortunately, I don't know the answer.
The other thing I would look at is SQL Server. Use SQL Profiler to analyze the queries being run by SSRS and see if that's a bottleneck. This is a fairly advanced procedure, so you'll need to hit the docs. There's plenty of info out there on how to analyze and performance tune queries, but this is somewhat of a black art that DBAs spend their whole careers perfecting.
You may be better off pulling in some professional consulting help if you're feeling daunted.