Problem: WiFi transfer rate is much slower than before or crash immediately after sleep wake up. This is a new issue to OSX10.11, it's ok on OSX 10.10.

Checking: Whatever you do, like turning WiFi on/off or switching to the 2.4GHz network,it wont be fixed. You must reboot your machine to solve the problem.

Fix: Disable "Wake for WiFi access" in "Energy Saver Preferences".