Internal SSD drives will load everything faster -- at least if you get a good quality drive.
It is certainly noticeable.
The speed of an SSD drive that connects through USB will depend on the speed of your USB port.
https://www.quora.com/Is-the-performance-of-an-external-SSD-decreased-by-USB-limitationshttp://forums.macrumors.com/threads/using-external-ssd-drive-with-usb-2-0.1177328/If you have a USB 3.0 port or connect the SSD drive internally to a SATA 6 Gbps port, you should see a difference (compared to an internal mechanical drive).
I wouldn't expect to see much difference if your computer only has USB 2.0 ports.
SATA -- 6 Gbps theoretical maximum speed
USB 3.0 -- 5 Gbps theoretical maximum speed
USB 2.0 -- 480 Mbps theoretical maximum speed