I wondered how much time running ForbiddenAPI checker, executed multiple times in each of ~40 modules, adds to the Maven build, so I ran mvn -DskipTests install, and then mvn process-test-classes, both pre- and post-patch. This excludes performing compilation, since I didn't mvn clean inbetween.
On my Macbook pro, with Oracle Java 1.7.0_13 and Maven 3.0.3, the best (fastest) of five runs of mvn process-test-classes for pre-patch: 20s. Best of five for post-patch: 22s. So it looks like this adds 2 seconds to build time, assuming a populated OS filesystem cache.
If I use sudo purge to clear the OS filesystem cache before every run, pre-patch best of five runs still took 20s, but post-patch best of five took 26s. So with an empty OS filesystem cache, this adds 6 seconds to the build time.
I think this slowdown is live-with-able.