I can usually set up my tent in a matter of minutes if need be. The inside will get a little damp, but it's not enough to be the end of the world. If I'm outside setting it up, I'll probably be saturated by the time I crawl inside anyway. Having a fast-drying synthetic sleeping bag is the key for me. Other options include looking for an undercover spot to assemble your tent (i.e. under a tree) before taking it out and putting the pegs in the ground. You could also wait for a few minutes to see if the weather improves -- even if it's just an easing of the rain. If it's really a sustained torrential downpour, you could start looking at other accommodation options, even if only for one night.
__________________
I am clinically insane. I am proud of it.
That is all.