I've found that the write pattern can have an effect on future reads. For instance, I've seen random 4k writes followed by 1m reads result in a somewhat lower

I've found that the write pattern can have an effect on future reads. For instance, I've seen random 4k writes followed by 1m reads result in a somewhat lower read rate than if the writes were originally done as 1m writes. The degradation you are seeing is rather extreme - it seems to be performing worse than a cheap SATA SSD. I'd say this is worth having a conversation with Kingston.