Why is Powershell's Install-Module so excruciatingly slow?
While this doesn't answer your "why", you might like to look at JustinGrote's high performance Powershell Gallery Module Installer:
.SYNOPSISHigh Performance Powershell Module Installation.DESCRIPTIONThis is a proof of concept for using the Powershell Gallery OData API and HTTPClient to parallel install packagesIt is also a demonstration of using async tasks in powershell appropriately. Who says powershell can't be fast?This drastically reduces the bandwidth/load against Powershell Gallery by only requesting the required dataIt also handles dependencies (via Nuget), checks for existing packages, and caches already downloaded packages.NOTESTHIS IS NOT FOR PRODUCTION, it should be considered "Fragile" and has very little error handling and type safetyIt also doesn't generate the PowershellGet XML files currently, soPowershellGet will see them as "External" modulesIt is indeed much faster.