USB 3.0 Sandisk Extreme 64GB Slow Speed

any idea if my sandisk extreme is deffective? and what to do with it?

Many things influence speed.  The machine’s hardware, the OS’s work load, the system’s security app/s, the USB drive’s OS configuration Removable vs Performance.  I doubt your drive is defective but it may take you some digging to find out why the speeds vary.