I’ve just noticed that some of the minified JS in the theme seems to be way bigger than it should be. e.g.
assets/js/vendor/bootstrap.js - 69KB
assets/js/vendor/bootstrap.min.js 434KB
It looks like the minified version is multiple copies of the library. I’ve quickly switched to a standard minified version (37KB) and it doesn’t seem to cause any issues.
flexslider.min.js seems to be affected in the same way.