I was about to return my Sandisk Extreme 64 GB because of the same slow speed issues, when I stumbled upon a solution to restore the original writing speeds.
Basically, as some other users have stated, this flash drive is a SSD on a stick, with no TRIM support. Therefore, if the flash drive is formatted ONLY ONCE, the speed levels will decrease due to FAT corruption. This is a MAJOR problem of this flashdrive if you plan to format it in the future. Sandisk’s official word is “DO NOT FORMAT” , but I think it is preposterous.
“Why is it a major problem?”, you may ask. Well, for those users who want to have a high-speed USB stick with no problems, the solution is not simple. The flash drive requires a deep format such as an ATA Secure Erase, which basically flashes all the clusters at once and restores the original status of the flash drive. A Linux distro that might come handy is Parted Magic (https://partedmagic.com),,) which includes many features to tinker with partitions, different kind of secure erasing systems and such. By booting your computer into Parted Magic using a Live CD/USB, you can ATA Secure Erase the Sandisk Extreme to bring it back to its high speed days. My flash drive went from 10 MB/s to 200 MB/s of writing speeds after following this procedure.
Word of caution: be careful what you do with Parted Magic. You can render your internal HDD and/or your USB flash drive useless (“bricked”). Do your search first and make sure you follow the tutorials in case of doubt. Or keep filing RMAs with Sandisk until you are satisfied. I have read about costumers who have returned up to 5 Sandisk Extremes due to this issue.
Another thing: do not try running Parted Magic on a Virtual Machine. By default, Parted Magic does not allow you to ATA Secure Erase USB-connected drives. If your internal HDD is not detected, it wil not allow you to open the ATA Secure Erase app at all. Once you boot into PM, it will detect your HDD and then you can proceed to NAND wipe your flash drive at your own risk.
Good luck and hope that my post can help someone…