It would be a tradeoff and it would really depend on the size of your code base. However in general Micronaut will be faster, with a smaller memory footprint, because it doesn't have Spring integrated in it, and tries to do a lot at compile time avoiding reflection. On the other hand, you will lose the convention over configuration, and trade the Grails plugins for Micronaut features. Grails actually uses Micronaut as its main context giving you access to some of its features.