You don’t avoid bugs by installing games on Program Files folder, mine is on F: drive (slave) same with all my Steam games and all work fine there.

My rig is lower than yours and i don’t have it that bad except on world bosses with too much skills being cast (5 fps), there was an old thread explaining about some tweaks to do on the user.xml file:

DrawActorPersent=“5” DrawMonsterPersent=“50” (note that the default values are 99, i changed mine to these, well not sure if they help in anything).

Go on Steam open the Bibliotca right click on the game go on Propriedades and on Definir Opções de Inicialização add -dxlevel 11 and press ok.

In game, disable all the FX on Settings, use the Low Graphics option and i would suggest you to turn off Show other Players Name.