MODSEC_31_ANTISPAM_URI involves an rbl lookup, for example.
There's nothing in the wiki or KB that I can see with any references to help with this.
And thats on purpose. The lesson we learned from the RBL rules was that some folks were just blindly turning everything on, and then reporting (or loudly complaining to the whole world on other forums, sigh... why do people do that when all they have to do is ask for help, its free afterall) that their sites were slow. And we don't want that.
So consider this ruleset a super duper secret easter egg for those that ask!
Now that you have asked, that will look up URIs in POSTS (as opposed to IPs) to see if they are known to be spammy. Its pretty spiffy. It will need a nice quick DNS server because its totally blocking code, it will look up the URL and will wait for the DNS to return a result before it will pass on the POST to the application to process. So if you have a slow DNS server, or the DNSBL is slow then the application will appear
to be slow. Neither the app, WAF or apache will actually be slow, they will just be waiting for a DNS reply. Twittling their proverbial thumbs and whistling into the wind until they get an answer, thumbs up or down.
The long term plan with all DNS based rules it to include a highspeed RBL-DNS with ASL to manage all the requests, and to store local copies of the zones where possible to "eliminate" the speed issues (there will still be a small penalty, because its still got to make the request, but it will be fractional compared to a remote request with a normal RBL).