My advice to anyone attempting The Welsh 3000s in a day would be:
Tackle this in mid-summer. The longer day and better weather made this an absolute pleasure for us but I can imagine the opposite being true.
Minimise breaks. Our final time was just over 15 hours, but my moving time was 10. So we spent nearly 5 hours stopping and starting. Synchronise your stops so that not everyone stops any time someone needs a 5 minute water break or layer swap. These stops add up fast.
Pack swap after descending from Crib Goch. There is no need to lug tents, sleeping bags and cooking equipment for the whole trip. Saving the weight allows you to move faster.
Hydrate and fuel properly. Saving precious grams is important, but the time wasted if you bonk or dehydrate and have to stop for 30-40 minutes to sort yourself out completely eclipses these savings. Rehydration salts and electrolytes are worth their weight in gold.
Steady on the uphill. Unless you’re an accomplished fell runner, attempting to run the uphills isn’t smart. You will waste a huge amount of energy for not much time saved.
Enjoy it. As cliche as it sounds, drink in the experience and don’t get too wound up on the time. We could have moved a lot faster but we enjoyed the day, took photos and had an amazing time outdoors which is what it’s all about after all.