Performance optimizations

[francoisdtOC]

Hi team,

I've gone ahead and added my forked code base with some basic optimizations changes here: https://github.com/francoisdtOC/moodle-quiz_downloadsubmissions/tree/Version-1.1-r2-optimized

We've noticed that the majority of the processing time is spent on creating copies of the user files for compression, so for now we have gone ahead and tried optimizing the rest of the code base.