"Why wouldn't I want to put the SSD drive internal to realize the max speed when in the OS?"
You were the one who originated this thread, with the question, "Is an external USB 3.0 drive any slower than stock internal?"
The answer is, "it depends".
The "stock internal" drive on a Mini is a 5400rpm 2.5" hard disk drive. If you were to add an external USB3 enclosure, with a 5400rpm drive inside, yes, it would be slower to boot. Once up-and-running, speeds would be equal (insofar as any "differences" would be imperceptible to the user).
If you add an external USB3 enclosure, with an SSD drive inside, it will be far FAR faster. Orders of magnitude faster than the "stock" internal drive. Boot times will be much quicker (about 15 seconds to the login screen). Up-and-running speeds will be indistinguishable from an internally-mounted SSD.
The advantage of adding an external SSD are that it is "plug and go" -- you don't have to disassemble the Mini and pull the guts out, and you don't have to buy another SATA cable (which can be more expensive than an external USB3/SATA dock). And you don't run the risk of accidentally breaking connectors on the motherboard -- there have been numerous posts here in the Mini forum about that.
The DISadvantages are (at least right now) that you probably can't update the SSD's firmware via a USB connection, and I don't believe there are any TRIM enablers yet that can access the SSD while mounted in a USB configuration.
When I first connected an Intel 520 series SSD to my 2012 Mini (using a plugable.com "lay-flat" dock), the speeds measured 410mbps for reads and 247mbps for writes.