Why is Wifi so bad on Yosemite 10.10.3

I am using a MacBook Pro (Retina, 15", late 2013). Since I installed 10.10.3 the machine didn't shut down. It ran until I hold down the power button to switch it off ( other topic ). Since 10.10.3 there's a very bad WiFi connection. I restarted my router many times. First the router than my Macbook. Same bad level. I need that for my job. Because quite often I work on different places.

Is there something I could do except reinstalling 9.x?

Regards from Berlin, Germany

bolshii