Here is the output of iostat when the problem occurred:
--snip--
Can you use another tools such as
iotop which shows drive usage in percentage when the problem occurred? if electrumx or bitcoin core have very high disk usage, then your HDD is likely the reason.
It should be able to do a lot more then the 1.3M shown. So if that is all it's pulling then the issue is elsewhere. If that is all it can do then that may be the issue.
Without knowing whether ElectrumX perform sequential or random read & write, i wouldn't say 1.3M/s is slow.