This is not “Java slow, Python fast” – I expected it to be the reverse – but rather that the developers who cranked out a messy Spring app somehow managed to cancel out all of the work the JVM developers have done without doing anything obviously wrong. There wasn’t a single bottleneck, just death by a thousand cuts with data access patterns, indirection, very deep stack traces, etc.
I have no doubt that there are people here who could’ve rewritten it in better Java for significant wins but the goal with the rewrite was to align a project originally written by a departed team with a larger suite of Python code for the rest of the app, and to deal with various correctness issues. Using Pydantic for the data models not only reduced the amount of code significantly, it flushed out a bunch of inconsistency in the input validation and that’s what I’d been looking for along with reusing our common code libraries for consistency. The performance win was just gravy and, to be clear, I don’t think that’s saying anything about the JVM other than that it does not yet have an optimization to call an LLM to make code less enterprise-y.