This workaround from the Debian iwlwifi wiki seems to have fixed the problem on my WiFi 5100 Link adapter:
Troubleshooting Slow WiFi problems when using Bluetooth Some devices like the 6235 do include a Bluetooth device in the same card, which may lead into radio conflict. Newer devices (7200 and up) try to solve them intelligently, but it is not the case of older ones. If your WiFi is slow when using Bluetooth, try adding the following to /etc/modprobe.d/iwlwifi.conf and reboot: - options iwlwifi bt_coex_active=0 swcrypto=1 11n_disable=8 https://wiki.debian.org/iwlwifi
