GW2 is pretty poorly optimized for modern era computers. Getting 30-40 fps around world bosses or big zergs is quite normal no matter how good your GPU/CPU is.You may gain minor improvement in FPS by upgrading your GPU. GTX 750 ti is getting quite old anyway at this point. You should be able to run GW2 at medium-high settings with your current build and having fps around 50+ or 30~ at high playercount areas.Turning ON "Effect LOD" and dropping "Character Model Limit" and "Character Model Quality" to high or medium have huge improvement in FPS during zergs.