Add comment re nvm_ensure_version_installed performance

nvm.sh

@@ -1347,6 +1347,8 @@ nvm() {
         return 8
       fi
 
+      # This nvm_ensure_version_installed call can be a performance bottleneck
+      # on shell startup. Perhaps we can optimize it away or make it faster.
       nvm_ensure_version_installed "$PROVIDED_VERSION"
       EXIT_CODE=$?
       if [ "$EXIT_CODE" != "0" ]; then