I also don't get how some have full squads with 15-20+ speed secondaries, even better they fit into their mod sets and aren't mix matched. And looking at their rosters they even have ridiculous mods on characters they don't use in arena.
I have to think there's more than luck to some of this. I've farmed mods exclusively for six months and got a few of the 'good' mod packs before they were butchered and doubled in price, and haven't gotten over 14. A single 14 speed secondary is the highest. Then a single 13, a single 12, a single 11. Quite a few 8s. And can't put together a good synergy mod set with my mediocre highest speed mods.
Months and months of mod farming to no avail. It would be extremely nice if they made it easier to obtain speed secondaries. It doesn't matter how much mod challenge farming you do, or how much money you throw at the game. If you weren't meant to have them you won't.