We’ve known for over six years (since before we started Drizzle) that the query cache hurt performance. It was for that reason that the query cache was one of the early things to be removed from Drizzle, it just didn’t scale on multi core systems that we were targeting.
So what about modern hardware? While working on MySQL 5.6 on POWER8, I enabled the query cache and ran a benchmark. Enabling the query cache reduced performance by an order of magnitude. I suspect the performance impact is even higher on MySQL 5.7.
My vote? Deprecate it. Deprecate it now, print a giant warning if it’s enabled and at some point just remove it. A single global mutex just doesn’t scale to 4 cores, let alone 24 cores at 8 threads per core.