To expand on what bigred says, the geared diff is an "open" design and as soon as one wheel looses traction, all the power will go to that wheel because that's how open diffs work. So, when you go into a corner and the inside wheel unloads, the car will loose some traction and hence it will be slower. Ball diffs are constantly under tension and whilst they will allow some rotational difference to take place between wheels, they will not slip (hence the term limited-slip diff) enough to allow the power to bleed away on cornering, which means the car is faster round a lap. The downside is they can be more maintenance intensive as they need setting up properly and occasionally need nipping up as the balls wear. Geared diffs are more a fit-and-forget item.
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