One of the few things I like about SIN is the effficiency of Changi. Never had I to wait more than a few minutes after clearning immigration to fetch my bag
If it was KUL perhaps i would be hesitant with Air Asia departing from altogether another terminal
go through immigration, hopefully no flights have arrived from southern countries that need a visa, choose your line carefully. then get your luggage go upstairs to the Jetstar check in . remember you can check in on line so do that.
it is a short flight to Pen anyway... Good Luck