The ones I know who have moved on quickly have "raided" their back catalogue of old friends. There was overlap as they started to distance themselves from their spouse and relied on the "friend" for support during the marriage breakup.
Anotger 2 who have met via OLD are not particularly happy, 5-10 years down the line but are now loathe to go through another break up.
If you were genuinely commited to a marriage or relationship then you don't switch off feelings quickly. You have to process loss and learn who you are again. I don't think this can happen when you are in the storm of a divorce so I see those dating whilst not divorced as a major red flag.
A new relationship is often used as a drug to soften the negative feelings that accompany a relationship ending. When the drug wears off, as it always does, those unprocessed feelings are still there.
Some people are afraid of their painful feelings, understandably as who likes to suffer but it is often just temporary and a transition leading to growth.
I think post relationships you can give out a vibe of needy for a relationship that attracts people or conversely so untrusting that repels people. Neither is good!
Getting emotionally healthy, learning boundaries and being financiamly & emotionally independent needs to happen and I just don't think it happens quickly for most people. You see it on MN all the time, we are moving in together because finances work better, we lurve each other and the classic "my children adore him" it is always a red flag