First of all: What is WordPress revisions? WordPress saves revisions and stores a record into database for each saved draft. This could be useful to roll back to any previous post or page version; quite interesting uh? This revisions of this system, however, threatens to massively increase your database sizes, thus slowing your website.
As described WordPress revisions can be useful, but it would be interesting to be able to limit them to a maximum number, saving your database health.