When you spin at high rpm it puts more load on your cardio system. Lower rpm puts more load on your muscles. Think of it this way, when you are pedaling at low rpm uphill, how long are your muscles in a contraction phase? Compare that to what you'd get at a higher cadence (say 60 rpm vs 85 or 90 rpm). At a higher cadence the contraction phase is shorter. I heard someone explain it by adding that at the high rpm you are bringing in O2 at a faster rate, thus helping to delay onset of lactic acid accumulation and accompanying muscle soreness.
It can take time to adapt to pedaling at 90-95+ rpm if you are not used to it.