External SSD for MacbookPro - get the right speed?

The external USB3 ports on the MacBook Pro Mid 2014 are definitely capable of 5 Gbps, and so it it is indeed meaningful to connect a SSD that way.

Note that the actual speed of the USB links changes depending on what you connect to it. So if you connect some older, slower device - such as a for example an old USB mouse - you would see the speed listed as being significantly slower. The same can happen if you connect an older USB hub, or you connect several devices to the same port of differing speeds.

Also note that the MacBook Pro has internal USBs for things like the keyboard, trackpad and camera. The speed of these is irrelevant in terms of the performance you'll see with externally connected devices.

Keep in mind that most every external SSD on the market is a common SATA SSD drive on the inside. There's two common flavors of SATA today, there's SATA at 3 Gbps and at 6 Gbps. Even with the 6 Gbps SATA interface on the inside the actual speeds of the data in and out of the drive will be closer to 4 Gbps, basically just fast enough that they didn't want to use the 3 Gbps interface and make that the bottleneck.

In other words the speed limitations will not likely be the 5 Gbps USB port. If it were then they'd use something like Thunderbolt instead of USB as the interface for their high performance external SSD drive.

Thunderbolt can get more speed than USB but there are few drives that can both plug into Thunderbolt 2 and offer greater speeds than USB 3.0. There are Thunderbolt 2 to SATA adapters available that will take USB out of the middle but it's not likely to give much in a speed bump, if it gives a bump at all, because unless you buy top end SSD drives they are still going to be common SATA 3 Gbps drives inside.

If you want more than the 3, 4, or maybe 5 Gbps that you'll get from USB SSD drives then investing in Thunderbolt 2 drives is a fool's errand, it's time to get a laptop with Thunderbolt 3 and USB-C.