External drive read and write speed

I have an XPS 7390 connected to a TB16 dock.  I have two WD My Passport drives connected to the TB16.  I am considering replacing one of the external drives with a SSD drive and had looked at a few alternatives.  Before buying the SSD external drive, I thought I should test the read and write speed I get with the WD drives as there are various alternatives available with SSD drives and I didn't want to buy one that was faster that I could utilize.  Using CrystalDiskMark I find that the read speed of both WD drives is around 40MB/s and write speed is around 34MB/s.  The Sandisk drives I am considering have speeds, depending on the model, up to 2,000MB/s.  

Can someone advise if (a) I should be seeing a higher speed with the WD drives and if so what may be reducing the speed, (b) what the potential speed I could get via the TB16 with a faster SSD drive would be.

Thanks for your help.