Goblin,
I am not sure what type of errors you have but it seems that your environment is not setup correctly. JSON is part of epesi, like PHP, Javascript etc. We use epesi every day ourselves, our clients - in real life, heavy usage, one client has over 350 users logging on every few seconds.
I suggest that you try our 30-day trial environment and see the difference: http://www.epesibim.com/freetrial
Our goal was to render a page in less than one second even with a slow Internet connection. We have clients using epesi from notebooks installed in cars connected via cellular modems (AT&T) and the performance is very good.
It looks like your experience with epesi is far from good. I apologize for the inconvenience. We try our best and from opinions of other users it looks like we are on the right path: http://www.epesibim.com/testimonials
However we know that we can not make everyone happy. There are other alternative products, commercial and open source - did you try them? Do they perform better on the same server?
We will be glad to answer any questions and concerns you have, but quite frankly our resources are limited. We split our time writing custom modules for our clients and at the same time improving the epesi framework for everyone with features requested by users facing real-life problems and scenarios. We are developing epesi since 2006 and what you see is a very stable and mature product. We try our best to answer forum in a timely manner, but you have to understand that our time is limited - even with a commercial products and paid support not every problem can be resolved immediately.
I am really sorry that it does not work for you.