well found it myself. Haven't been at it for a while, but today just wanted to figure it out.
Seems the problem was in the properties of the wireless-N 100 adapter.
In the properties - advanced tab there is a option called:"Ad Hoc QoS modus" which you can change to "WMM on" or "WMM off".
This was in the "off" position. When turning this on my wireless speeds immediately went from 5Mb/sec (not Mbit!!) to 9,8Mb/sec copying big files (20Gb+) on my local network. All other properties are in the default position.
So case closed. I'm one happy user now!! : )