Without knowing whether ElectrumX perform sequential or random read & write, i wouldn't say 1.3M/s is slow.
Yes but...most external drives are crap.
However, from my non scientific testing over the years, mostly 2.5" portables, they tend to be consistent crap in terms of the drive but not so much in the controller.
From the ones I use most USBs will never see a large difference between sequential or random reads / writes. The USB controller that talks to the drive itself is the weak point and you never get to the limit of what the drive and it's controller itself can do.
Just what I have seen. YMMV.
-Dave