After discussing this with a few individuals, the general consensus is that speculative JS parsing gives us a pretty big win despite the asan shutdown leak in bug 1634641. Also, a lot of this infrastructure is soon to change with Stencil so we will revisit this issue after that has finished. In the meantime, turning on JS parsing as soon we fetch scripts should return some of the wins we previously saw.

Enable speculative JS parsing of external scripts as soon as they are fetched.

https://hg.mozilla.org/mozilla-central/rev/861ee9635ee2

== Change summary for alert #26734 (as of Wed, 19 Aug 2020 07:29:35 GMT) ==

Improvements:

5% raptor-tp6-twitch-firefox-cold windows7-32-shippable opt 254.07 -> 241.23
4% raptor-tp6-twitch-firefox-cold windows7-32-shippable opt 260.88 -> 251.15

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=26734

== Change summary for alert #26770 (as of Fri, 21 Aug 2020 11:09:53 GMT) ==

Improvements:

21% microsoft-support android-hw-g5-7-0-arm7-api-16-shippable opt cold 2,212.43 -> 1,747.54
20% bbc ContentfulSpeedIndex android-hw-g5-7-0-arm7-api-16-shippable opt cold 3,062.33 -> 2,436.17
17% microsoft-support android-hw-p2-8-0-android-aarch64-shippable opt cold 1,256.12 -> 1,044.11
13% espn loadtime android-hw-g5-7-0-arm7-api-16-shippable opt cold 5,273.96 -> 4,570.71
12% microsoft-support fcp android-hw-p2-8-0-android-aarch64-shippable opt cold 1,386.46 -> 1,226.08
11% microsoft-support fcp android-hw-g5-7-0-arm7-api-16-shippable opt cold 2,766.00 -> 2,474.92
10% microsoft-support FirstVisualChange android-hw-g5-7-0-arm7-api-16-shippable opt cold 2,760.67 -> 2,474.08
10% microsoft-support SpeedIndex android-hw-g5-7-0-arm7-api-16-shippable opt cold 2,856.42 -> 2,567.75
10% microsoft-support PerceptualSpeedIndex android-hw-g5-7-0-arm7-api-16-shippable opt cold 3,035.50 -> 2,745.00
[...]

For up to date results, see: https://treeherder.mozilla.org/perf.html#/alerts?id=26770