To make visual editing of transclusion parameters possible, we need to improve the performance of the current implementation, as enabling parsoidConfig.addHTMLTemplateParameters currently causes complex pages to take a big time to produce.

One approach, proposed by @GWicke, is to expand templates in parallel. From a message to the parsoid email list:

1. add an API entry point for transclusion-to-token expansion; return JSON-serialized tokens. (Eventually, once we know a transclusion is balanced, we can actually just expand to a DOM here.)

2. experiment with calling this a) if some parameters need to be expanded to HTML, or b) if the template is in some list of expensive enough templates, or c) always.