NPM3

Why npm 3 installation so slow?

What happens when command "npm install" invoked?

The answer just lies in the manual page for npm install command. Run npm help install and see the "ALGORITHM" section.

npm help install
# ...
# load the existing node_modules tree from disk
# clone the tree
# fetch the package.json and assorted metadata and add it to the clone
# walk the clone and add any missing dependencies
#   dependencies will be added as close to the top as is possible
#   without breaking any other modules
# compare the original tree with the cloned tree and make a list of
# actions to take to convert one to the other
# execute all of the actions, deepest first
#   kinds of actions are install, update, remove and move
# ...