Codier/angular.js — comment on commit 655bf7110df4a351c42a93ff806952658699063d
File: src/service/scope.js line 347
By mhevery:

JavaScript JistInTime compiler. or JITer. The jitter is what converts JS, which is normally interpreted, into machine code, which is way faster. But when you do a delete it relegates the object forever into the world of interpreters, and you lose the benefit of JITer.