I wasn't involved in the switch, but having extensive knowledge of the code base at Tumblr (used to work there for 3 years) as well as extensive knowledge of PHP (have been a PHP contributor for ~13 years), I can say that this was likely >95% directly related to the switch to PHP7. Many of the patterns used in Chorus (inhouse framework Tumblr is based on) and in the codebase in general would very much benefit from the changes in PHP7 internals.