Pneumothorax-- can it come on slowly?

Sort of... It is estimated up to 10% of spontaneous pneumothoraces are asymptomatic and/or unrecognized. It is possible for someone to develop an asymptomatic pneumothorax that progresses and becomes symptomatic hours, even days later.
Yes possible. Yes some times lung can leak slowly and the reason to take sequential chest x-rays to see the progress.