I wouldn't spend any time bench marking the SSD. It's likely just fine, and you'll be asking yourself why it benches fine but doesn't work in your real world scenario.
SSDs are not better at large, sustained data writes, especially low capacity or cheap SSDs. If you need high amounts of sustained writing capacity, at high speeds, you're looking at a different tier of SSD altogether.
In short, you exceeded the drive's internal cache that it uses to maintain fast write speeds, which on a Kingston drive like that is very likely around 8 GB, and as a result, you got stuck writing at the underlying TLC speed of the flash chips, rather than the SLC/MLC buffer write speeds. Don't overrun the buffer and the drive should do just fine. The buffer will flush after a period of inactivity, so it isn't as though the drive will be permanently hobbled.
Slow writes aside, SSDs have very high, uniform seek rates, fast reads, and good throughput. The great seek times are what make them so responsive.
If you want a larger buffer, some higher priced drives have 16 GB while others still are equipped with 32 GB. This is also dependent on the capacity of the drive, as capacity is being taken from the flash chips and partitioned as SLC/MLC rather than TLC/QLC to facilitate the higher write speeds.