After r30578 it is now possible to list multiple parameters to &list=usercontribs&ucuser= ... this also creates the possibility for very efficient pseudo-CIDR lookups, eg:
&list=usercontribs&ucuser=22.214.171.124|126.96.36.199|188.8.131.52|184.108.40.206|220.127.116.11| ... |18.104.22.168
A typically tested /24 takes only .4 seconds on en.wp, and goes back to at least 2002. However the URI limit at Wikimedia seems to be about 4000 characters, slightly liming the usefulness of this (and using format=json in a <script> precludes using POST).
Could this behavior, then, be supported natively? Either via CIDR formatting? say up to /24 for normal users, and maybe as far as /20 or even /16 for bots (which could take over a minute):
Or perhaps some sort of prefix parameters: &ucuserprefix=1.2.3. (though this may not be optimal)
Whichever is most efficient to the current design of parallel user lookup, while allowing a shorter URL.