Apple - Tool to objectively test performance various external drives on my Mac via USB & Thunderbolt

Blackmagic Disk Speed Test

If you're interested in read and write speeds there is an app called Black Blackmagic Disk Speed Test available on the App Store. This doesn't have a database to compare with others but can give you actual speeds for your internal and external drive.

AJA System Test is another commonly used tool to benchmark disk performance for the Mac. It offers graphs and a text report of performance instead of just a dial indicating transfer speed like Black Magic.

If you want to test random I/O, which neither AJA or Black Magic do, you can use the tool fio like you can on Linux. You can expect the IOPS to be much higher on an SSD than on a HDD in this case. fio is available from Homebrew