At least back in the POWER8 days those z/Linux systems were the fastest you could buy, and IBM was super happy to let you overclock them: their reps told me that was just more CPU sales for them.
While the mainframe may be old and out of fashion, it did have the capabilities that we are rediscovering in Cloud, containers, VMs and all...
Many comments tout the uptime and reliability of the "mainframe", I'd argue we have many counterexamples built on Linux. Building a product with this level of reliability on Linux is expensive but still cheaper than a mainframe and the various support contracts, IMHO.
I started out working with an IBM AS/400 in college and eventually worked for a company that ran their business on one. Eventually market pressure forced that company to move to Windows servers accessed through Citrix from thin clients. In my opinion, this didn't make much material difference: it was still complicated, people on the floor complained about Citrix+Windows just as much as they did the old IBM terminals. Hardware costs and support contracts were less expensive but the cost of the software was much, much more expensive and the organization no longer had the ability to change any substantive functions. Just sayin', moving away from a mainframe isn't necessarily a clear win either.