It's easier for me because they're a tool I use for my trade but I use ceramic tile nippers which I think are a better choice especially on a high value card.
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My main concern is not damaging the card and I could do it faster if I wanted, it usually takes about 30 seconds per slab including Beckett slabs.
With the nippers you don't have to worry about getting close to the card because you only nip the very outer edge. I usually do the two long edges and one short edge and then you can usually just open it with your hands.
To do this you just hook the upper jaw on the raised edge and pull back towards you to snap mall pieces off.