Some time ago, after an update (Android/Kernel?), when I turned wifi on, literally, everything ran slowly and jerkily. I have now turned off 'Network notification' and immediately my beloved Nexus ran as fast with wifi on as with it off.

Settings > Wi-fi > three dots top right > Advanced > Network notification (off)

Android version: 5.1.1 Kernel version: 3.1.10-gf5d7b8b