We are using SSDs in a JBOD configuration. I can do some testing to check iowait and disk utilization today.
The more immediate issue we're facing is that we can't run queries for longer than 2 minutes. The team that owns this cluster is pointing to a Kibana timeout (issue crossposted to Kibana forums) they say they can't modify until 7.4. Does that sound correct? I believe I've ran >2 minute long queries on previous Elastic stacks without issue..