lyx [1] is great for live editing of latex .
Set the instant preview Tools>Preferences>Display>Instant Preview if you want automatic latex rendering, although the editor rendering is nice as it is.
For something that is more of a calculator, SymPy [2] with sympy.init_printing() would be rendering latex automatically in any Jupyter notebook.
Specific tools I created for my own use for leveraging the above is pypad [3] together with calcpy [4] (%load_ext calcpy)
I think PyPy usually has slower startup time.
Need to try to defer some of the imports to be done in the background, see how good is the outcome.
Feel free to open an issue in github.