Oh my, my webhoster has the deflate output filter disabled by default, that enables gzip compression of outgoing content. This is important for huge xml/json data from webservices that travel over mobile networks and easily reduces used bandwith to up to 10%.
Putting this in my .htaccess did the trick:
AddOutputFilterByType DEFLATE text/plain AddOutputFilterByType DEFLATE text/html AddOutputFilterByType DEFLATE text/xml AddOutputFilterByType DEFLATE text/css AddOutputFilterByType DEFLATE application/xml AddOutputFilterByType DEFLATE application/xhtml+xml AddOutputFilterByType DEFLATE application/rss+xml AddOutputFilterByType DEFLATE application/javascript AddOutputFilterByType DEFLATE application/x-javascript
You can analyze the traffic with Wireshark, Firebug or use an online tool:
Make sure, your mobile app sets the Accept-Encoding of the request accordingly.