> The startup time is also a real problem, as you really want to be able to scale up pods quickly.

I was learning a bit of spring last week and a spring boot web application, generated via the web interface boots in like 800msec:

    ...
    Initializing Spring embedded WebApplicationContext
    Root WebApplicationContext: initialization completed in 339 ms
    Tomcat started on port 8080 (http) with context path '/'
    Started DemoCourseApplication in 0.746 seconds (process running for 1.012)
    ...

Reusing my experience from other technologies... I'd say the issue might be in whatever you're doing in your initialization and/or how much stuff you're loading.

Looks like the core spring is decently fast, to me.