it takes too long to export

Hello everyone. I'm trying to export a movie but the program tells me that it's going to take about eight hours to do it, which seems strange to me considering that the actual movie lasts twenty minutes. All I've done is inserted a few black mattes with titles and some music. Oh yeah, and I lowered the quality to fifty percent so that the final movie takes less place. But anyway, I don't see why it should take so long?
Cheers!