The current version of NPM (3.5.2), and possibly other versions, have an interesting issue. Having the progress bar enabled actually slows the install process by around 20%.
Running the command
npm set progress=false will disable the progress bar, and thus bypass the code causing the issue.
If you are interested in the in depth analysis of why this is an issue, there is a comment on this thread that describes it in incredible detail.
It also appears like this issue will soon be fixed in later versions of NPM.