Why does it need to be fast anyway? How many programs will this one-language-programmer be writing that needs to be fast? Programs are generally constrained by

Why does it need to be fast anyway? How many programs will this one-language-programmer be writing that needs to be fast? Programs are generally constrained by just one thing: either CPU, network bandwidth, network latency, user latency, disk I/O and so forth. Why focus on the CPU? Is this hypothetical programmer really going to be writing another git or a program to fold protein molecules, or is he more likely to be writing glue, user interface or one-shot data manipulation programs? Similarly, multi-threaded programming is hard. If this hypothetical programmer is only learning one language (and thus, by the admission of the article, not very good), why is this suddenly a requirement?