Fix elapsed time check
Summary:
To match the code it was refactored from it should be checking the code
since the last time we checked calcSpeed, not since the job started.
CCBUG: 402665
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D18158
(cherry picked from commit cd2f67c39b25de026390bfe2bc1c7aa269f78ccb)