It's likely that Java will be higher performance than python, but since you haven't said what concurrency you're targeting, it's hard to assess how much hardware you'll need to handle the load. If suggest choosing the language and told that feel best for you to start with, and measure how well the system handles the load before optimizing your language and framework.
Note that Knative will automatically scale the number of containers based on requests-in-flight, so you can potentially handle high throughput by running many containers with a small amount of parallelism per container of that makes more sense (e.g. due to Python global interpreter lock).
For Java, a natural choice might be Spring.
For Python, I've been happy with Flask.
In either case, I'd recommend choosing a stack that you're comfortable with (or, one you always wanted to learn if it's a hobby project) and measuring to see if it performs well enough.
welcome to 2023!
Mid-2022, the Knative project launched the func tool, which allows you to avoid needing to choose things like web frameworks, and instead to simply get started writing your function from a simple template. The function tool will then add the necessarily libraries without you needing to get into the details of Flask / Spring / whatnot.
