Thanks, I narrowed the issue to two specific problems that cause VAR{} and @{} to be slow when used intensively with large objects: currently these syntaxes make a copy of their value before returning it to avoid unwanted mutation in #{} or JS{} scripts (see this post for technicals). I will adjust this in next version so that only the scripting functions getVar() and get() behave this way.
Now the best solution regarding performances would be to handle all the magic in a custom module and use the /EDIT command to modify your widgets or even on the fly. This way you can keep the session tight with as few widgets as possible and have most of your javascript in the same place which is much easier to maintain.