The scoping rules are by design and match .ipynb workflows in the case of multiple documents, so we're unlikely to change it.
The render latency of quarto is definitely higher than we'd love, but we have a plan and have been steadily improving it. Quarto 1.4 is generally about 20% faster than 1.3, and we have performance regression infrastructure to not let us slip on it.
For my personal workflow (others may differ), compiling to html is only done once at the end of a session, and the latency wouldn't matter if it could execute like a script. Weave.jl^[1] has a great feature called `include_weave` which has the features I like.
But take my feedback with a grain of salt. I generally just save things in folders and compile a pdf separately with many tables and figures.
[1] https://weavejl.mpastell.com/stable/usage/#include_weave