Unreal engine has one huge FPS hog while displaying things (well tbh every game engine out there has it at this same spot). It is multiple overlaying and transparent sprites, in simple words particle effects can kill everything fast.

And all spells, effects in TERA are done as particle effects, and they are over the top because asian games love to do it this way. I played with "cascade" (ie. particle effects) in unreal and UDK for years, even best computers cannot handle more than 2000-3000 at once. And from look of spells in TERA one effect uses around 100 sprites. So you get more than 20 people and even most powerful computers starts to choke.

My tip: play with advanced settings in graphics options. Lower effects to minimum when you about to go for huge PVP event. It was same in every MMO, still remember DAOC sieges when you actually could lg in trough walls. TERA is no better or worse than rest.

Does anyone think flashing my BIOS would help with compatibility?

I'm surprised that NO ONE has suggested one of the MOST obvious potential problem.... the internet connection.