This Kingston A400 SATA SSD is a low end budget model which does not have the same performance levels of some other SSDs such as the Crucial MX500 series which is available for just a little bit more money. I personally stay away from most of the low end budget SSDs because the majority of them have serious issues. Even Crucial's low end BX500 series SSD is terrible and tends to overheat easily. Many low end SSD models are only made for light workloads with minimal amounts of writing.
You can try unchecking "Put drive to sleep when possible" in the Energy Saver System Preferences and you can try enabling TRIM on the SSD. Let the iMac sit without doing any work to see if the SSD's performance comes back after the SSD's internal garbage collection routines kick in. Budget SSDs tend to slow down after writing lots of data such as when installing macOS, restoring from a backup, or after installing/updating software. All of these things also trigger Spotlight to index the drive as well which will slow down the performance greatly and will interfere with the SSD's internal management routines which are needed to get the SSD back to its standard performance level. Leaving the Mac sitting on the Apple boot picker menu is a way to help the SSD to get back on track since the SSD will have power to do its internal maintenance while not having macOS continuously interrupting the SSD.
Edit: You can check the health of the SSD by running DriveDx. Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper. Unfortunately DriveDx and similar types of apps don't always properly report the health of an SSD, so manually examining the SSD's health report is usually best as it can also sometimes reveal clues.
