They are several times faster than light and time slows as you near light speed if you were to go as fast as the Z fighters it would be as if you were outside of time itself.
Actually, I'd think you'd go into the past.
Sorry, Marvy, but it doesn't work that way. First off, Kid Buu doesn't go for the absorb unless he's facing someone with godlike ki, which Doomsday doesn't have. Second, those "time restraints" are only when they gather tremendous amounts of power. Buu would still need to gather power to have enough to take this team, and he would still have to wait to do so.
Doomsday does, however, have godlike strength, endurance, and durability, all in one package. A tempting opportunity for Buu.
Also, Buu pretty much always has his power ready when he needs it. The only time he "gathers" power, from what I've seen, is when he's pouring his reserves into a ki blast, not his own body.