ElectroServer 5.1 was released today with tons of updates that you can read about in the release notes. Aside from added examples and documentation, the biggest enhancements are noted here.
The bandwidth used for the most common message types was reduced significantly. It was already light weight but we were able to optimize it much further. In addition, messages can be automatically compressed if they are over a certain size threshold.
We tuned the low-level messaging layer to achieve unprecedented performance. We've done testing to more than 100,000 messages per second with very low CPU impact (on EC2). This level of performance gives the server-side portion of your game plenty of room for its own logic to be executed.