lazynvm.sh

# lazyload nvm
# all props goes to http://broken-by.me/lazy-load-nvm/
# grabbed from reddit @ https://www.reddit.com/r/node/comments/4tg5jg/lazy_load_nvm_for_faster_shell_start/

lazynvm() {
  unset -f nvm node npm npx
  export NVM_DIR=~/.nvm
  [ -s "$NVM_DIR/nvm.sh" ] && . "$NVM_DIR/nvm.sh" # This loads nvm
  if [ -f "$NVM_DIR/bash_completion" ]; then
    [ -s "$NVM_DIR/bash_completion" ] && \. "$NVM_DIR/bash_completion" # This loads nvm bash_completion
  fi
}

nvm() {
  lazynvm 
  nvm $@
}
 
node() {
  lazynvm
  node $@
}
 
npm() {
  lazynvm
  npm $@
}

npx() {
  lazynvm
  npx $@
}

In zsh, I have a different setup in my ~/.zshrc that not only lazyloads nvm and node, but also any globally-installed package

# Add every binary that requires nvm, npm or node to run to an array of node globals
NODE_GLOBALS=(`find ~/.nvm/versions/node -maxdepth 3 -type l -wholename '*/bin/*' | xargs -n1 basename | sort | uniq`)
NODE_GLOBALS+=("node")
NODE_GLOBALS+=("nvm")

# Lazy-loading nvm + npm on node globals call
load_nvm () {
  export NVM_DIR=~/.nvm
  [ -s "$(brew --prefix nvm)/nvm.sh" ] && . "$(brew --prefix nvm)/nvm.sh"
}

# Making node global trigger the lazy loading
for cmd in "${NODE_GLOBALS[@]}"; do
  eval "${cmd}(){ unset -f ${NODE_GLOBALS}; load_nvm; ${cmd} \$@ }"
done

updated for CLI npx and bash completion

Thanks @christophemarois, I've been hunting for a solution like this one!