According to his description, 28000x14000 is the resolution of his environment map; rendering speed should be pretty much independent of that parameter. Still 6 fps at 1920x1080 with 15 bounces is kind of cool.
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Here's an article of interest about efficiently sampling discrete distributions: http://www.keithschwarz.com/darts-dice-coins/. Of particular interest is the Alias method which can sample in O(1) with O(n) memory. If I'm not mistaking, Jacco's technique is described in "Simulating a Loaded Die with ...