Bo-Christer Björk and David Solomon have self-archived "The Publishing Delay in Scholarly Peer-Reviewed Journals."
Here's an excerpt:
Using a stratified random sample we studied average publishing delays in 2700 papers published in 135 journals sampled from the Scopus citation index. The shortest overall delays occur in science technology and medical (STM) fields and the longest in social science, arts/humanities and business/economics. Business/economics with a delay of 18 months took twice as long as chemistry with a 9 month average delay. Analysis of the variance indicated that by far the largest amount of variance in the time between submission and acceptance was among articles within a journal as compared with journals, disciplines or the size of the journal. For the time between acceptance and publication most of the variation in delay can be accounted for by differences between specific journals.