We're having the same problem.
Overall troughput of a media agent has dropped with 50%.
Aux copies were > 4 Tb/hr went < 1Tb/hr and vsa (san) backup copies were about 600Gb/hr and now about 200 - 300.
We've opened a case, and that is stretched to the limit
We are about the enter registry keys for the performance maybe that will help.
Of course, nothing has change, only the continuesly patching of microsoft/hp patches.