Follow-up from T191921: mwscript rebuildLocalisationCache.php takes 40 minutes on HHVM (rather than ~5 on PHP 5) - as far as I can tell, l10nupdate is calling mwscript directly, which is still using HHVM by default. For consistency with scap, it should explicitly use PHP 7.
As a side note, it would be nice if T133913: Completely port l10nupdate to scap was worked on, which would have prevented this issue.