When running a full scap sync as part of train we shell out to MediaWiki to use mwscript rebuildLocalisationCache.php. When using php5 this took between 5-10 minutes. Using HHVM this takes 40 minutes.
But we can't use php5 anymore after https://gerrit.wikimedia.org/r/#/c/425026/
And there are blockers to using php7 (T172165)
so it seems like we're between a rock and a hard place and that we have to figure out why hhvm takes so much longer otherwise scap sync will take an addition 30 minutes every time.