60 shards is too much IMO.
1 shard is enough here.
spinning disk
Switch to SSD drives.
Also pasting again one of my questions:
What is the typical response (the full one please) of a "slow query"? And what was the request?
Please read this about how to format.
Also note that "6.3.0" has been built more than a year ago.
Lot of improvements have been done in the meantime. Could you upgrade to 7.2.0?