What format are you currently using, what size are most files on the drive, which revision is this drive, how full is it, and have you used the DiskPart or DiskUtility menu to fully “clean” or “erase” using these apps?
File size and type can account for over a 100x variance in write speed. For example, I have a SanDisk 128gb Extreme Pro, which is their flagship flash drive. When writing larger files like videos, write speeds of over 200 MB/s are common. However, if having to write many smaller files, write speeds between 1-5 MB/s are common.
A partial explanation of this is due to native USB 3 controllers having poor write speeds in regards to small files, and while SSDs can maintain lightning fast speeds even when the drive is almost filled, USB 3 flash drive controllers do not have quite this resiliance. Some makers have resorted to putting SSD controllers inside flash drives, at the expense of a larger form factor & higher price tag…the Corsair GTX is an example and it has 4k write speeds that are literally 5+ times faster than my Extreme Pro.
I do not know of any tools that are SanDisk specific, but using any of the standard benchmark software will allow you to test the speed and compare it relative to others who own your same drive. Depending on the controller of your drive, it may or may not be able to support SMART - if it does, a program like CrystalDiskInfo will pull health statistics. Where as the 128gb Extreme Pro does support SMART, I am unsure about the Extreme.