What are the specific model numbers of the disk drives involved? Please bear in mind USB 3.0 transfer speeds depend on controller driver support and raw system specs. An average of 70MB/s-90MB/s is within expected performance considering Apple’s implementation of the interface:
Please bear in mind 60MB/s is the maximum sustained output of USB 2.0 on an optimal environment without hardware, bridge (Driver), software (Firmware), or system interference. Most systems peak around 20MB/s.