consider that when Reck procs you wont usually be at 0 swing CD. more often you'll be mid-swing. also consider when the mob dies, you might kill it on the first proc. so the 2.0 speed/8 secs is a little theorycraft utopia.
Reck is similar to SoC in that it's unpredictable. if you were able to chain Reck indefinately then a 4.0 weapon would destroy all. but given that it all averages out, 2.0+ will give you the same results over the long term.
(for those wondering, this is all number crunching, actual results will vary and this doesn't take into effect all the other pally abilities or specific gear)
"(For all following cases, we will assume a 50dps weapon, with just the swing speed differing)
For the first case, we'll take a weapon with a 1.5 swing speed. 4 swings of that weapon would take 6 seconds, well within the 8 seconds of Reckoning. Overall damage would be 4 x 75 (50dps x 1.5 speed) = 300 damage.
Next, we can look at a weapon with a 2.0 swing speed. 4 swings of that weapon would take 8 seconds, fitting exactly within the 8 seconds of Reckoning. Overall damage would be 4 x 100 (50dps x 2.0 speed) = 400 damage.
For the last of the simple cases, we'll take a weapon with a 4.0 swing speed. 2 swings of that weapon would take 8 seconds, so we would 'waste' the last 2 charges of Reckoning. Overall damage would be 2 x 200 (50dps x 4.0 speed) = 400 damage.
Moving onto a more complicated example, a lot of people have been championing 2.2 speed weapons. With a 2.2 speed weapon, 4 swings would take 8.8 seconds, whilst 3 swings would take 6.6 seconds. So when Reckoning procs we are either going to get 3 or 4 swings, depending on our current swing timer. Basically, if Reckoning procs 0.8 seconds or more after a swing, then you'll get 4 attacks, otherwise you'll only get 3 attacks. The maths here is a bit more complicated but still easy to figure out. Damage for 4 swings would be 4 x 110 = 440, whilst the damage for 3 swings would be 3 x 110 = 330. The *probability* of getting 4 swings would be 1 - 0.8/2.2, whilst the probability of getting 3 swings would be 0.8/2.2 (ie. the chance of being inside 0.8 seconds of a swing timer of 2.2). The numbers work out to be about 64% for 4 swings and 36% for 3 swings. To work out the average damage, you'd multiply the potential results by the probability of them occurring. So 440 x 64% + 330 x 36% = 400 damage."
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