The Aspect Ratio (A/R) is what determines if it makes a ton of boost down in the low rpm or if it makes the boost up higher in the RPMs. Just like a manual tranny. If you start out in 1st, you can't go all that fast, but if you try to start out in 4th, it will barely make it out of the starting line. Fixed A/R turbos are this compromise, you can have the boost at starting off or high up in the RPMs. Your choice. The variable vane turbos take care of this but it takes a computer to do it.
Dave