[rwmj]

The article answers this in the first few paragraphs. Mainframes are extremely reliable and have high I/O bandwidth (and relatively poor CPU performance). You can run Linux on them - Red Hat ship and support RHEL for s390x - but most of the time they run z/OS along with z/VM. https://en.wikipedia.org/wiki/Z/OS

[iamnotwhoiam]

But 5ghz is an incredibly high clock speed. Is the cpu hampered in other ways?

[rwmj]

I use z14 at work and it's not especially fast for single thread stuff. For the applications that typically run on these, performance of the entirely separate channel controllers (I/O) is what matters.