I don't know if it was due to high ATK IVS, but my Zekrom in my original playthrough of white 2hko'd Reshiram with fusion bolt+dragon claw. If you really wanted to speedrun on white, I believe the only way to reliably do this is to save+restart and rng manip in the middle. But this would create more time than it saved, so it's still always more beneficial to attempt on black. (even if the difference is like half a second... any time counts.)

That being said, I'm broke, and will be doing my attempts on white since that is what I own.