You should count yourself lucky if you get the total advertised storage capacity, regardless of speed.
There was a scam years ago with storage (SSD, hard drive, USB sticks) that sold you lots of gigabytes, but that internally had tiny storage and wrapped around any write over the actual capacity back to the start of the drive. If you checked the storage superficially it worked, but once you you were filling that disk with data it would start to overwrite the previously stored data.
Current scam is to relabel used hard disks as new:
Investigating authorities have unearthed a workshop in which used hard disks were converted into new goods. Models from Toshiba and WD were also found.
www.heise.de