PiCrate is a stand-alone ruby wrapper around vanilla processing, that does not depend on a vanilla processing install. It uses the java environment that is already installed on raspbian-stretch (much lighter weight than vanilla processing with its bundled jvm), however it does require an install of JRuby. A PiCrate sketch is pretty much just ruby code (there is no fancy pre-processor step cf vanilla processing), and there are many editors (eg vim) and ide (eg geany) that support coding in ruby (cf vanilla processing that requires its dedicated ide, that is java based, and hence memory intensive grew out of JEdit).

Vim is incredibly lightweight but mainly suitable for power users, whereas geany is more suitable for those requiring a GUI, but is still quite lightweight. Further you can launch sketches directly from both.