Sandisk Extreme 64GB - Slower write speeds after system format

Just off the phone with Sandisk support. Their USB sticks and memory cards are ment to be formatted. 
So, format them if you want for as much as you want. You won’t break it by doing so. 

Just like SSDs, no way you can physically break your USB disk or have it’s performance degraded by formatting it (however obviously you will loose data on it). Well, not as long as you format it 24/7 for a couple of years (wear out memory chips by writing data more times than it’s been designed to handle; and those are pretty generous nowadays anyways). 

So happy formatting, people!

If the drive performs slowly (or does not work at all) then you should contact the support.