#Alfie, What kind of lag do you experience, lag from the network (actions in game [like opening vault, using skills/NPCs] have huge delay) or lag from your PC (low frame rate in game [wings, text, anything other abnormally slow])?
In first case please close every other applications that use your internet connection, make sure networking devices [like router/modem] restarted [unplug them for 30 secs, then plug all of them one-by-one back], and try to use
WTFast as final solution.
In the second case there are so various factors that can cause lag (even for the most expensive gamer PC...). Please hit
Ctrl+Shift+Esc to open the task manager, navigate to the
Processes tab, and order descending by CPU, then make a SS when you experience the lag, and share here, since it can give a better overview about the your problem.
Also make sure your PC is not infected with some malicious software, scan it asap. [It's possible that your CPU/GPU is mining Bitcoing for some hacker, or your PC sends hundreds of spam emails every hours, or DDoSing some innocent server]