I confirm the upgrade of the nginx increase the performance of nodebb. (index page was often realy slow / and some times crashes) The latest stable version of nxing has resolved this problems.
To upgrade on ubuntu:
sudo apt-get install software-properties-common
sudo add-apt-repository ppa:nginx/stable
sudo apt-get update
sudo apt-get install nginx
npm install installs everything in package.json but we have different dependencies based on what database you pick during node app --setup (redis, mongo, level). Usually just running npm install is enough but in this case there were changes to the database dependencies.