Source of database performance issues found
Additionally to the upgrade of the SSD few days ago, I had the opportunity to upgrade the database from MySQL 5.6 to Percona 5.6.
Upgrade yesterday evening and today morning was not that smooth, as some libraries were outdated, therefore I needed to upgrade them first.
Finally now all works fine.
Current stats: Open tables: 952 Queries per second avg: 924
In Sum, the WN8 calculations hit the server hard:
Seems, I need to spend more time on this making it work more efficient.