You are actually programming in Groovy, with all the plus and minus it entails.
Regarding the execution speed, IDEs not able to autocomplete, no guarantees about the format that IDEs can easily parse, and it often breaks between releases.
Then to compensate for lack of speed, you need to have a running daemon and an in-memory build cache.
If your build generated sources (such as via antlr or some other lexer/parser that has a maven plugin) at build time, you know that it's likely to be done at the 'process-sources' phase.
If you wrote a custom gradle build script (or indeed, any other custom scripts), then you cannot be sure when that generation of sources is going to happen.
Maven forcing you to design your build around maven's fairly strict set of phases (and build lifecycle) means you confirm, and any tooling can compose as long as they also conform.
