He goes slow. He doesn't want to hurt Bruce, and slow seems like a good pace anyway. It gives him time to ease Bruce into excitement and hold him there, rather than rushing past all of the better sensations.

A little pause, and Tony slips the finger in deeper, curling it slightly once he's inside. He makes sure to keep himself slick. The start is always somewhat uncomfortable, but he's not going to rush and cause Bruce pain. If anything, the curl of his finger should give him some extra pleasure. A new sensation that he can melt into and learn about.

"You're just too sweet," Tony tells him. He kisses him again on the side of his erection, sliding his finger deep and then sliding it out again. He takes his time, giving him a few steady thrusts like that so Bruce can get used to the feeling. "Think you're going to make it?"