Thanks for the feedback. It will be challenging to offer both extensibility and performance safeguards. One idea I had was to do some form of performance evaluation of each theme/plugin and show a score or impact grade to the user when they are enabling it. A bit like how Google Fonts shows you the impact of the web fonts you've selected on page weight.

I know a few people who have had problems with speed with their Wordpress sites, will tell them to check out Cachoid.