That said, I've got Grails apps running in production just fine, and to whatever degree noticeable speed is an issue, it's almost always down to database indexing.

As Hamlet said, I don't think execution speed is the primary reason people aren't using Groovy. Personally, I think all the "alt.java" languages are fighting for attention from the few people left in the Java/JVM arena who are truly interested in exploring new ideas/techniques. Many Java devs who were interested in getting better years ago left for other platforms, (and many of those people will likely jump to new platforms every few years out of curiosity more than anything else).