I can only imagine, as this is two accounts acting differently on the same computer, there has to be a difference in settings. Or maybe your girlfriend has armor/gear/kuso that is glowing or sparkling and you don't?
I suffered a lot of lag and freezing on my previous computer that was 9 years old and making some changes in the settings could really make a big difference.
Things in settings to try on your girlfriend's account:
- Put the settings on mid or low
- Glow FX: off
- Max FPS: 30
- Depth Effect: off
- Title: off
- Helmet: off
- Cape: off
Also note the what chat tab is active. When the combat chat is active and you are in combat it's moving very fast and showing lots of information on DMG given, DMG taken and so on. The faster the chat moves, the more lag you could experience.