Firebug could help you to analyze some relevant statistics. Open Firebug and navigate tot the "Net" tab. When you load a page, you will see the size and time to load of your request.
Edit after comment:
The mendix client consumes the biggest part of your data, when you load a mendix app the first time. After that, caching should take over. You said you're building an app mostly used by a tablet. It's reasonable to build this in Mendix 4, so that you can use mobile forms, which are already optimized. Is that an option?