For the comfort of the pax, any speedbrake causes vibration and noise, so it makes sense to use it all for a short period than a little for a longer time. The vibration is much the same with partial or full speedbrake. It does not do much until it is all the way out, and even then it is not much good, it is only a spoiler after all, and since it blows down at high speed and at low speed it has only a small effect, I always use all or none.