Hi, thanks!
I work only on the draw function with the REPL mode, while having some variables declared beforehand (you can’t declare them on the fly, at least not in the Processing IDE). I test my performance quite often, and when I see I need a variable in it, I add it as a global variable (I used to have variables names v1, v2, v3 etc., but was not really handy).
For the output, I use OBS and overlap the coding window to the rendering window, quite hacky but does the job quite well 
Here you can find the theme of my IDE: