HDM15 very slow full backup

HI, I'm doing automated periodic backup (full) of two partitions. In the past it took not more than hour and half to finish but since some moment processing time extended to approximately 10 hours, which means that computer must be turned on all this time. I consider this non normal speed of processing (the problem is with 2nd partition as first one is done in less than a hour). While the size of final snapshot is not much bigger than in the begins (~120GB).
Is any explanation for this rapid...

HDM15 very slow full backup