Hi Daniel,
After weeks of stuffing around with WP Supercache and Autoptimize, I switched to Cache Enabler -which works well with Autoptimize- (https://wordpress.org/plugins/cache-enabler/) which solved all my problems except that javascript issue. Async-Javascript took care of that straight away.
I am now scoring 100/100 for speed on mobile and desktop on Google PageSpeed and 92% on Yslow perhaps because I am not using a CDN.
The important thing is that mobiles can actually load the page now which was not happening before, even when Google and Yslow were giving me high scores. That Javascript just stopped the front page loading on mobiles.
It is certainly worth trying.
Hakim