It's important to distinguish between cases where it is slower, and cases where query plans change. MySQL 5.7 changed the cost-model significantly, so while it

It's important to distinguish between cases where it is slower, and cases where query plans change. MySQL 5.7 changed the cost-model significantly, so while it will be better on most queries, there are cases where two-wrongs-made-a-right; and edge case queries may be slower. To mitigate this (as it was somewhat expected), MySQL 5.7 also introduced new query hints and server-side query rewrite. (In the case of the link you provided, OP responded later saying 5.7.10 improved performance again.)