I also experience this maddening delay running PlantUML version 1.2019.11 in Eclipse.
In lieu of any other method to stop a new diagram generation with each edit I make, I find entering "##" on a line by itself works well since it results in a syntax error, which causes PlantUML to stop generating diagrams. When I am done editing, I simply remove the ##. Should I not have placed the ## at the beginning or end of my document (or near the section on which I am working), the Find /Replace feature quickly locates it for me.
Hope this helps.