What is "usernoted" and why is it eating my CPU?

Usernoted is a process that is responsible for notifications on OS X.

The instructions from this post solved usernoted sitting at 100% CPU on my Mac, even though they were intended to only fix stuck Notification settings:

1. Open the Library folder in your Home folder.
2. In the Library folder, open the Application Support folder.
3. Locate the folder named NotificationCenter. Drag this folder to the desktop.

4. Next, open the Terminal application. Copy and paste each line of these commands into the Terminal window, in order. Press return after each line:

   cd `getconf DARWIN_USER_DIR`
   rm -rf com.apple.notificationcenter
   killall usernoted; killall NotificationCenter
   

5. Close the Terminal app.

6. Restart your computer.

For me, the problem started after I had upgraded from Yosemite to El Capitan: usernoted was constantly sitting at 100% CPU and nothing I tried (force-quitting usernoted, checking if Growl or Bark are installed, booting into safe mode, SMC reset, running all the maintenance and cleaning scripts in Onynx, etc.) seemed to help until I tried the steps listed above.

I had same problem, at last, I found that it was because of Webpack I'm using in a frontend project. Webpack is a command line tool. Many terminal-notifier processes are consuming much CPU resources.

Once I quit Webpack, usernoted exits, and CPU become quiet!