Centrino Ultimate N-6300 slow to connect to WiFi network

WiFi connections from my laptop became unstable after I upgraded it to Ubuntu 19.04. To get them to work, I had to turn the WiFi adapter on and off several times, which quickly became annoying.

The following attempts to fix the problem were unsuccessful:

• disabling IPv6
• disabling power-saving on the WiFi adapter
• setting bt_coex_active=0 during kernel loading to disable Bluetooth/WiFi interference protection

Eventually the correct solution was to disable 802.11n with the following procedure:

1. Edit or create a configuration file under /etc/modprobe.d (e.g. iwlwifi_custom.conf).
2. Add this line and save the file.
   options iwlwifi 11n_disable=1
3. Reload the iwlwifi module.
   sudo rmmod iwldvm
sudo rmmod iwlwifi
sudo modprobe iwlwifi
4. Connect to the WiFi network.