A slow passenger is only a problem because of the low number of exit hatches. The bottleneck is the number of exits, not the slow passenger.
This is tested with volunteers before certification, see e.g. [3] for a video of the certification test for the Airbus A380. The details [4] are quite interesting: a third of "pax" must be over 50, three life-size infant dolls must be carried along, etc.
Given those certification requirements, a slow passenger is very much a problem.
[1] http://www.faa.gov/news/fact_sheets/news_story.cfm?contentKe... [2] http://adg.stanford.edu/aa241/fuselayout/far801.html [3] https://www.youtube.com/watch?v=XIaovi1JWyY [4] http://www.ecfr.gov/cgi-bin/text-idx?SID=4fcf34b24a7a7963642...