I assume you're using USB 3.0 everything? (otherwise you'd max out at about 35 MB/s) My USB 3.0 external enclosure pretty much maxes out the speed of the hard drive itself, so 70-110MB/s for the "low power" drives.
What are you copying, and what are you using the measure the speeds? Note because of the filesystem, copying small files is WAY slower than copying large files. For instance, if you're copying a load of <1kB files, expect less than 10 MB/s. You should run HDD benchmark like the following to be able to test the drive and what attaches to it:
Pick whatever you like - HDTune:
HD Tune website
CrystalDiskMark:
CrystalDiskMark - Software - Crystal Dew World