you can control the weight of your pack / gear, but sometimes you can not control the "fast" aspect of Light and Fast. I learned this in Alaska last spring when the route my partner and I climbed was supposed to take us two days, and ended up being 4 of the longest days of my life. Its hard to move fast through chest deep or higher sugar snow, can't control the conditions and sometimes up is down...
I would say "efficiency" is more important than being fast. For example stopping to brew up on route and climb hydrated will help your body stay much warmer, and you will be able more more efficiently than a shivering cold slow dehydrated body would... Trust me I leaned from my mistakes last year we each lost 15lbs in that 4 days...
Knowing the conditions, and knowing when too and being ok with turning around is also a key factor in Light and Fast tactics. Still learning that the hard way.