It's possible that as its still a fairly new install that bigbox is still caching all the relevant images, clear logos etc. I don't no which theme you are using but some are much more media rich than others and will need a bit more horsepower for a smooth experience. Bigbox actually uses a gpu if you have one installed also and that can make a difference also. Have you tried force caching your images? There is a option for it in options/image cache/force populate images, this can take a while depending on collection size but will cache all images to your hdd. This should smooth out performance for you.