well first make sure the lag is not from your pc and second ,this is a beta so that mean they test the server config to, im sure that if the lag come from them they will fix that soon, so you don`t need to worry about this, is normal in any beta test game to be a bit lagy
If you have Hardware lag (mouse movement is slow) rather than Network lag (Rubber-banding or moving back a few steps when walking forward) then you should make sure the resolution for the game is on the lowest setting (800x600) and that your graphical settings are set to low.
if you still experience the mouse lag then unfortunately your machine might not be powerful enough to run the game.
If it is network lag (Rubber-banding) then I would suggest you disable your antivirus and/or firewall (windows and other if you have another). it can be very frustrating when you are excited to play a game but you can't but the best thing to do is keep cool.
Once the game has been out for a while I'm sure someone will come out with a custom config setting to enable the game to run on lower end machines or even Ubi might possibly enable for lower detail levels for the same purpose.