This paper describes recent developments of the Siebog agent middleware regarding performance. This middleware supports both server-side and client-side agents. Server side agents exist as EJB session beans on the JavaEE application server, while client-side agents exist as JavaScript Worker objects in the browser. Siebog employs enterprise technologies on the server side to provide automatic agent load-balancing and fault-tolerance. On the client side this distributed architecture relies on HTML5 and related standards to support smooth running on a wide variety of hardware and software platforms. Such architecture supports rather easy, reliable and efficient communication, interaction, and coexistence between numerous agents. With the automatic clustering and state persistence, Siebog can support thousands of server-side agents, as well as thousands of external devices hosting tens of client-side agents. Performed and presented experiments showed promising results for real life applications of our architecture.