I assume those seconds are mainly JVM startup time. The existing ob-plantuml doesn't support sessions, so it evaluates every code block in a new instance of plantuml. Seems to me like should be possible to use the `-pipe`-option for plantuml so that you can send multiple code blocks to the same instance via stdin and get the images back via stdout. That's probably what VSCode does, but it doesn't seem like anyone has bothered to write that for Emacs so far.
