Add to the /boot/boot_config the following
[all]
dtoverlay=miniuart-bt
core_freq=250
see also https://forums.raspberrypi.com/viewtopic.php?f=28&t=138631&start=100#p939040
Add to /etc/ssh/sshd_config
IPQoS cs0 cs0
e.g with command
echo "IPQoS cs0 cs0" | sudo tee -a /etc/ssh/sshd_config
restart sshd:
sudo service ssh restart