I was tired of long boot times and slow program launches on my late 2014 Mac Mini. Even with 16 GB of RAM, the thing was pokey.
The 2014 Mac Mini wasn't the most difficult Mac to work on that I've ever encountered, that honor goes to the pre-Unibody Macbook Pro. Ugh. Replacing the hard drive in the Mac Mini required pretty much completely disassembling the computer, though. It was tedious, but, thanks to the iFixit guide, it wasn't difficult. No hitches in re-assembling, either. After installing the Crucial SSD, boot time dropped from a couple of minutes to under 20 seconds. Programs that took a minute or more to launch now launch in seconds.
Just follow the iFixit instructions. They were flawless.