I checked a few sites in Alexa and question2answer.org is very popular. That may not be all in the Q&A section though. Many of the other sites in the directory are very popular in their own countries.
frageee.de (popular in Germany)
undegasesc.info (popular in Romania)
zalicz.net (popular in Poland)
surak-zhauap.kz (popular in Kazakstan)
need-help.org (popular in India and worldwide)
pokemondb.net (mine, popular worldwide)
Also to answer the performance question, see gideon's answer here, it's been tested against a huge database. My site is pretty popular and I've had no problems.
