I doubt switching the backend to Spring Boot would reduce the speed of development much. You simply want to avoid having to reinvent the wheel, and plenty of languages have mature frameworks for that.
This sort of Java is almost all about boilerplate and patterns. Two or three decent engineers taking a few days to plan for the rest of the project can write 90% of the huge amount of boilerplate that Spring Boot needs.
One might say not as agile as starting to code from day one, but I'd rather take an extra week than risk a major rewrite in the middle of development.
You won't ever need to do a major rewrite - the dynamic language web backends scale up to big levels (see shopify using rails for instance), and at that point your infrastructure will be challenging regardless of what language you are using
One might say not as agile as starting to code from day one, but I'd rather take an extra week than risk a major rewrite in the middle of development.