Hello Andrzej_P,
We understand you have wireless speed issues with the Intel® Centrino Wireless-N 6235.
There are some settings you may want to verify in order to optimize the 802.11n connection:
http://www.intel.com/support/wireless/sb/CS-030709.htm
http://www.intel.com/support/wireless/wlan/4965agn/sb/CS-028449.htm
http://www.intel.com/support/wireless/sb/cs-025343.htm
You may also update the Intel® Centrino Wireless-N 6235 adapter driver which is available here:
http://www.intel.com/support/wireless/wtech/proset-ws/sb/CS-034041.htm
Please note the Intel® Centrino Wireless-N 6235 is a dual band adapter which works at 2.4 GHz and 5 GHz. However, since your router and most consumer devices work in the 2.4 GHz band and there are only three non overlapping channels some interference may be expected. In this case we recommend moving your system away from devices working in the same frequency or attempt to use other non overlapping channels as described below:
http://www.intel.com/support/wireless/wlan/sb/CS-031509.htm
You may additionally test the connection with a different router model.
I hope the suggestions help.