Fragment shader invocations are minimum 2x2 pixel blocks. Portions of those 2x2 pixel blocks outside the primitive represent wasted computation. Sliver triangle

Fragment shader invocations are minimum 2x2 pixel blocks. Portions of those 2x2 pixel blocks outside the primitive represent wasted computation. Sliver triangles, with their long edges and small sizes, tend to increase the number of wasted helper invocations.