I do not care to argue the merit or folly of the underquilt but it does suggest to me that you have to fill the volume of the space with warm air in order to stay warm. And that’s going to take a bit. On the other side there are foam and air pads that will act as insulation and the results are immediate.
Speaking of pads I’ve recently experimented with full and torso length cell foam pads. The 1/8th inch pads did not offer much protection at all. The 1⁄4 inch pads were much better, however, [a] they had to be hydrophobic in case they fell out of the hammock. [b] pliable enough to wrap around the body [c] and slippery enough that you can roll.