My particular testing was with an SSK SD300, which is roughly 500MB/s up and down. I have benchmarked this and confirmed it meets its rating.
I have thought about buying something like a Team Group C212 or Team Group Spark LED, which are rated at 1000MB/s. The 256GB version of the C212 is available on places like Newegg and Amazon for around $27 USD at time of writing, but they make variants as high as 1TB.