I think the Kingston A400 SSD is a low end budget economy model that can be as slow as a hard drive (I may be wrong and confusing the model, but I know many of the Kingston SSDs are low end). It can be very difficult trying to determine which SSDs are low end budget economy models which are extremely slow and which ones are actually decent SSDs. The manufacturers are making this more & more difficult as they try to hide the deficiencies of the newer SSD technology from users since the new SSD technology is slower and wears out faster. If you want an inexpensive SSD that performs well, then use a Crucial MX500 SSD (stay away from the BX500 series as it is a slow drive & overheats). It is a good compromise on price & performance.
For a 2009 Mini you may want to use an OWC Mercury Electra 3G model instead for better compatibility. The OWC 3G SSD uses a SATA II controller which means it has no problems negotiating the SATA Link speed with your Mini which also uses a SATA II controller. Most other SSDs use a SATA III controller which should be backwards compatible and work with the Mini's SATA II controller, but in reality many SSDs have issues auto-negotiating the SATA Link Speed so you wind up with intermittent problems.
I have never installed a Crucial MX500 SSD into a 2009 Mini so I cannot say how well the Crucial SSD will work. I've never had any issues with the Crucial SSDs in 2010 Macs (at least the few we have upgraded). We use the Crucial SSDs in most of our organization's upgrades with a few exceptions where we use the OWC 3G SSD (just to be safe).