With a 2014 Mini, you might want to install an internal SSD. I have not done this (mine already has an internal SSD) but the installation looks very simple
This will give you about 700MB/sec performance but an external USB SSD is going to be more like 400 MB/sec. As @IngerMan indicated, his results come from a newer Mac that has USB-C ports which your 2014 Mini does not have. This is what I get from the internal SSD on my 2014 Mini
And this is a Samsung T3 connected via USB. The difference is pretty substantial, and if your Mini is "swapping" to disk due to low memory then you want the fastest possible SSD
Not really applicable to the OP, but FWIW, this is what I get with a Samsung T7 external SSD connected to my 2018 Mini via USB-C
This will give you about 700MB/sec performance but an external USB SSD is going to be more like 400 MB/sec. As @IngerMan indicated, his results come from a newer Mac that has USB-C ports which your 2014 Mini does not have. This is what I get from the internal SSD on my 2014 Mini
And this is a Samsung T3 connected via USB. The difference is pretty substantial, and if your Mini is "swapping" to disk due to low memory then you want the fastest possible SSD
Not really applicable to the OP, but FWIW, this is what I get with a Samsung T7 external SSD connected to my 2018 Mini via USB-C