Zend certified PHP/Magento developer

What can bottleneck a CPU for single-threaded applications? [closed]

For sake of installing packages, manipulating files (copy, paste, delete, load, save), booting up applications, search (file explorer), and running Python code and generic applications – what can bottleneck single-threaded CPU performance, besides hard-drive and RAM speed? Are there caveats to upgrading only the CPU?