Unfortunately the performance is not fine. I gave also a wrong number for the data per second - in fact we are talking about up to 100 KByte per second with a base load of maybe 20 KByte/second.
I guess I will need to move the logging into a seperate thread - so that the application will keep its speed.
I moved the logging into a seperate thread.
It was way too slow using the logfile write - I had only about 600.000 lines of constant back log.
Now I have moved to a standard file and the performance is ok.