I ran into the same problem, using --verbose as peterh mentioned showed me the source of the problem: I am behind a proxy, which uses its own certificates for https-connections.
According to user “Mletter1” on https://github.com/npm/npm/issues/8872 the issue is quite easily solved by using http:
npm config set registry http://registry.npmjs.org/ --global
And voilà, it’s fast again. Of course you should only do this, if you’re ok with cleartext npm infos over the net 😉