Hi Rainer,
BlazeDS or LCDS depends on your application requirements, if you are using traditional http, webservice or amf remoting request and response model then there is no difference between LCDS and BlazeDS because you are still restricted by the your OS, hardware and bandwidth capacity for processing number of requests per second.
Use of LCDS depends of your application requirements e.g. if you are looking for Real Time Messaging Protocol (RTMP), Reliable communications, Data conflict resolution and synchronization, data push and Enterprise document integration such as PDF capabilities then LCDS would be the choice.
Please refer the comparison chart from
http://www.adobe.com/products/livecycle/dataservices/compare.html to make a right decision.
Thanks,