If it is 10 copies of the same thing the Burner will be getting hot and will be physically slower.
If you are making copies of Movies the CPU will be hotter than normal the the Circuitry on the M'Board will be slowing down the CPU to prevent it overheating.
Basically this will be because of a Heat Issue and under normal circumstances there is nothing you can do but leave the system to cool down to regain performance.
If you persist in making copies the system should start to get even slower till it gets tot he stage where it can not run any slower and it will continue to run at that speed till you finish whatever it is you are doing.
You can get burners that are designed for Bulk Coping but these are more expensive than the normal domestic ones and you can change the CPU Heatsink & Fan to something bigger but again this is going to cost you and it is likely to increase the noise.
If you re making copies of different movies the HDD can also become heavily fragmented so the Data Transfer is slower running Defrag after 10 or so copies may not be a bad thing here either.
Col