Wifi Freezing iPhone

When I connect my iPhone to a wifi network the phone becomes unresponsive and slow. It happens anytime I get in range of a wifi network that I have previously used. Just walking passed a cafe that has a wifi network that I have previously used is enough to freeze the phone. The only way I can get the phone to work again is to swipe down and wait about 30 seconds, press to disable wifi, wait another few seconds for the phone to recognise this and once wifi has been disabled and disconnected from any network the phone works perfectly fine.

Any ideas?