I was gonna write a piece about this, but since you started it, I'll enlighten you, if you can bare the boring math for a minute or two.
So you're looking for the prized legendary 6* speed boots from the speed set ya? Hey who isn't? We could also talk about the real Holy Grail of them all, that same piece but with Crit rate, Crit Damage, Accuracy and any of the % stat, you'll fit them on the appropriate champ... but this is a dream within a dream, so let's stick to the more accessible, any Lego 6* boots with speed on speed set.
Accessible? You think so? WRONG! Here are your chances, say at dragon 20 cuz we're talking accessible for every player, right?
1) Do 6 runs, you get one pair of boots
2) Do 7 sets of #1, you'll eventually get one pair of boots with Speed on them. We're at 42 runs
3) Do #2 13.33x, so that you get a legendary drop of those same boots. We're now at 560 runs
4) Repeat #3, 3.70x so that the piece actually is a 6* piece. We're now at 2071 runs.
5) And now, to top it all, you need them on the speed set.. crap, so how many more? Well, there's 9 sets, so without that sham 2x boost, you'll need 18,643 runs, at 16 energy each = 298,296 energy.
But hey, lucky you, we're in the 2x drop period, so your chances improve from 1 out of 9 to 2 out of 10. because they put one more speed set drop in the bag, giving 2 for speed and 1 for every other set. So this comes out to 1 in 5. So you'll need 10,357 runs or 165,720 energy.
Do you think you can run all those in 3 days? LOL nope. Without the boost, if your runs take about 3min each, stop reading this because you need to go in there and start farming right now, for the next 6 weeks, non-stop, 24 hours a day.
Good Luck!