yes it will speed up growth, but lettuce already grows pretty fast.. at least hydroponically.

the only time i would consider giving leaf greens 24 hour light is at the very early seedling stage, because it takes the longest period for the plant to develop (root mass) and once they have develop several true leaves they really explode in growth where you can give them less light because they grow so fast, it's almost unbelievable really.. i've trimmed a lettuce and 3-4 days later it's a full head again. the way i see is, why bother increasing electrical costs to shave off a
few days of growth. i've grown leafy lettuce from seedling to palm size
leaves in about 20~ days and this is at about 13-14 hrs light.. good light is key for good growth so less light (hrs) is more. at one point i was growing around 30 heads of lettuce under a 4ft 4 bulb T5 fixture putting out 20,000 lumens. from this setup i could feed a family of 4 lettuce every single day of the week and still have some to give away.