Maybe your neighbor can no longer leach your wireless signals.
WPS is a feature for lazy people. All it does is allow you to not key in a password. It has massive security holes in it and should have been removed from all router since it can not be fixed but then lazy people would not buy routers I guess.
It really should not have affected your connection speed. Make sure you did not also disable the WPA/WPA2 which is the actual encryption. If you have encryption on then you should leave the WPS off.
The only reason I could think of is that someone has been using WPS to easily connect to ur router. Now that you have it disabled, that person can no longer get on ur network and eat up ur bandwidth.
It's better to disable WPS altogether and authenticate using ur password. You can save ur password on the devices u use, and if you do that, you will have no need for WPS at all. Extra tip:
tip: the WPS password can be guessed using a bruteforce attack, and there are new methods that make this thing so easy that we should disable WPS forever to secure our networks.