The low-end Mac Mini 2014 has poor performance, but I knew that when I bought it.
I bought a bottom-of-the-line Mac Mini 2014, intending to upgrade the internal drive to an SSD and add external storage using the drive from an old Time Capsule with a bad power supply. I ordered the replacement SATA SSD and empty MiniStack from OWC (macsales.com); sadly they still don't have a PCIe upgrade kit for this Mac Mini. Replacing the hard drive with a SATA SSD, tearing apart the Time Capsule, and putting it all together was quite easy with the guides on iFixit (though I did ruin the Time Capsule bottom cover because I just didn't care). The only downside is that you have to completely disassemble the Mac Mini to get to the hard drive.
Upgrading the 2014 Mac Mini yourself is a lot cheaper than getting an SSD option through Apple, and the end result is a Mac Mini with pretty good performance, especially compared to a spinning hard drive. Unplugging the power supply from the logic board during disassembly is a bit tricky, and one screw on the hard disk is a bit hard to reach using the 64 piece driver due to the shape of the hard drive chassis.