How are you measuring speed here? You should try to measure it. Try copying some files to the external drive and note the speeds.
You will never get close to the maximum speeds published for the standards. SATA III is 600 MBytes/sec and USB 3 is 500 MBytes/sec. Remember that the published specs are always in Mbit speeds, not MByte, as the bit-to-byte conversion varies. I get about 25 MBytes/sec transfer speed between two interrnal hard drives for simple file copying and a max of about 120 MBytes/sec using backup programs. Disk testing programs give read speeds for my drives of 150-200 MBytes/sec. depending on file size. You get higher transfer speeds if you copy large files. My backup image programs zip the data into 4GB files, which transfer at the highest speeds.