wlan connection slow

Hi,

since a few days I have big problems with the WLAN-connection of my late 2011 13" Macbook Pro that leads to very slow Intenet Connections and Slow SMB-Server access.

In Detail: If I will open a Webpage it will take 50 times longer then from my iPad or Windows PC; and I run an Windows 2012 Server as an Data Pool, if I will access to any Remote Hard disk the Mac will need 5-10 sec. to load a Folder (the Windows PC loads it directly after the "klick").

Intristingly, If I am connected via LAN (T-based 1000 Mbit Gigabit Ethernet) everything works extremly fast.

Does anybody have a Idea to Solve this Problem? I run Mountain Lion in the latest Version, nothing has changed the System since or before the Error occured.

Thanks....