Regarding performance issues (in general), a very detailed guide has recently been prepared:
I would say that this is a very extensive topic and getting into formal troubleshooting can be a very complex and time consuming process.
So it might be worthwhile to evaluate if there is any topic/aspect of interest, do some testing, etc.
At the very least, it will be an extremely useful reading… 
Definitely, it will need a dedicated thread if this will go forward (I assume that sporadic problems, only when copying files, are not such an important issue).