For minor possible changes, you can look at adding the options:

BLOCKSIZE = 65536

BUFFERCOUNT = <something appropriate - maybe try 12 for 4GB RAM or any 32-bit, 128 for 64GB RAM>

MAXTRANSFERSIZE = 4194304

Other than that and the advice above, adding more disks to source or target drives and/or checking for filesystem level fragmentation on the source or the target are options.