The only reason to do 12/12 from seed, is if your ONLY space to grow (even seedlings) is your flower room (and has something flowering so it can't be changed from 12/12).

You will not be able to harvest any earlier by doing 12/12 from seed. Your harvest will be diminished, and your time will be equal, making your yield per day worse. Much worse.

18 or more hours of light per day until the plant is at least 6 weeks old will flower at the same time as 12/12 from seed. But the plant will be much more developed, and have a higher potential yield. I don't know of any plants that reach sexual maturity before 6 weeks, so you don't lose any time by vegging in a normal light scheme for at least that long. Most plants will keep vegging for another couple weeks in the 12/12 before they start flowering, because they won't reach sexual maturity until 2+ months.