After updating to Ubuntu 18.04 Bionic, I found my RALink RT3090 wireless card using the rt2800pci driver introduced latency. This was most noticeable as a delay while typing over SSH.
This is because power management is enabled on the card, which puts it into sleep mode and it takes some time to wake up again.
You can see the power management state of the card with:
sudo iwconfig
Amongst other things, this will show:
Power Management:on
You can test whether disabling power management solves the problem with:
sudo iwconfig DEV power off
Where DEV is the device name (eg: wlan0, wlp3s0)
If this solves the problem, then apply the setting on boot. Edit the file /etc/NetworkManager/conf.d/default-wifi-powersave-on.conf and change:
[connection]
wifi.powersave = 3
So it reads:
wifi.powersave = 2
