Unable to copy a .mpg file from NAS to computer

I copied yesterday my 700 MB from internal HD to My Cloud EX2 Ultra. It worked fine with my GB local network.
Today, I stored some video files which I manage to convert from .mov to .mpg. The convert software was running on my computer, it wrote directly on the NAS on a Z: share. Then I would copy the resulting file to local HD or USB key. This later works properly. I am able to play the .mpg file straight from the NAS. But when I want to copy, the explorer starts in a calculating sequence, then copy a little amount of data and stops, restart au calculating sequence and fails with a networt default.
I am quite disapointed with that.

I have found the solution by myself. I wrongly set a big size for the frame jumbo parameter as I mostly use big files. It made the whole device awfully slow. I return this parameter to normal (ie stop) and every thing is OK. Even ,the access to the file in the NAS is quicker than with my old local HD!
Thank you for attention.