turbine wheel, bearings, motor's VE, motor's intake cfm, motor's combustion chamber, manifold design, timing, blah blah... theres 1000's factors.
98% of which will only effect you in first gear in a race. after that, you're above 5k 99% of the time.. if you're not- its tire size and gearing issues- with a load placed on the eld, which will induce spool. , and lag is non existant if you know how to drive and can shift quick, the turbo won't loose but a couple pounds or so give or take all of the above.