MySQLd was behaving fine prior, no issues, just upgrading for currency and any few features.
I have a 16Gb dedi, with InnoDB buffer pool given 8G. With MySQL 5.6 this was great, and mysqld sat around 20% CPU utilisation and approx 50% total memory utilisation.
Upgrade to MySQL 5.7, ensure same /etc/mysql/my.cnf config (upgrade didn't make changes) and ensure that 5.7 mysqld is using /etc/mysql/my.cnf still.
No, my mysqld is trashing CPU (400+ %) and memory usage is only 17% of total RAM.