Yes, db4o is essentially single-threaded. He uses one big lock to protect all his operations. It is easily displayed in code.
How to maintain bandwidth? Well it just doesn't! db4o has never been built for applications that perform multiple simultaneous operations (e.g. webapps). It is really built into desktop / mobile apps. It cannot handle server load. For high throughput, you need to use larger databases.
Besides a single thread, db4o also offers no good concurrency control for detecting conflicting changes in various transactions.