Back in the day mainframes had an I/O advantage because they offloaded a lot of I/O processing to coprocessors (e.g. a minicomputer would just have one processor while a mainframe would have one main processor and one or more I/O coprocessors). That advantage appears to be gone now because I think they are using normal PCIe cards and normal cores as I/O coprocessors (which you could also do on a non-mainframe server).

Of course, mainframes are also faster simply because they are more expensive (e.g. 100 PCIe slots are obviously faster than 10 slots), but that's not an intrinsic advantage (in theory you could develop a $10M x86 server — call SGI if you're interested).