In the offline version of GlowScript VPython (the GlowScriptOffline zip file), after running many programs execution slows down, apparently due to there being too many WebGL contexts. The offline version should do what the online version manages to do, which is that when you end execution the WebGL context needs to be terminated. See untrusted/run.js and lib/glow/WebGLRenderer.js.
In the offline version of GlowScript VPython (the GlowScriptOffline zip file), after running many programs execution slows down, apparently due to there being too many WebGL contexts. The offline version should do what the online version manages to do, which is that when you end execution the WebGL context needs to be terminated. See untrusted/run.js and lib/glow/WebGLRenderer.js.