It requires a perfect storm of just shy of 100% used memory and a lot of mmaped io. In that case the mmaped can get shunted to a handful of pages (or even one page) and so you lose all ability to have any block io larger then one page size. And every page of memory involves a fully blocking io request. It’s most certainly real for certain workloads (databases are common).

"Feels weird" when she logs on to investigate serious performance problems that she's getting paged for.