Slow WIFI on iPhone 3GS after update to iOS6
Reset your network settings, to clear all remembered WIFI networks. This will force it to create a new ones. You will have manually connect to the WIFI network again, and if required enter its password again afterwards. 

You can reset the network settings by selecting:

Settings -> General -> Reset -> Reset Network Settings

Also what can help is trying to clear the history of your Safari browser, you can do that by going to the Bookmarks -> History and then press the Clear button on the lower left hand corner of the screen.