In the Google docs, they've identified a pagespeed filter that will load the script asynchronously:
ModPagespeedEnableFilters make_google_analytics_async
You can find the documentation here: https://developers.google.com/speed/pagespeed/module/filter-make-google-analytics-async
One thing to highlight is that the filter is considered high risk. From the docs:
The make_google_analytics_async filter is experimental and has not had extensive real-world testing. One case where a rewrite would cause errors is if the filter misses calls to Google Analytics methods that return values. If such methods are found, the rewrite is skipped. However, the disqualifying methods will be missed if they come before the load, are in attributes such as "onclick", or if they are in external resources. Those cases are expected to be rare.