Speed is less and less 8d

I´m want to copy files from another NAS to 8D. I only get around 20mbit :frowning: Even when i copy that files first to my Mac SSD i get 20-25mbit.
I have no SSD installed yet to the 8D. In the beginning there where 127mbit. Seems that the drobo is going slower when more data is on the 8D. Somebody had the same effect?

I have a 5D3, and my Drobo regularly grinds to a halt, for no apparent reason.