It's worthy, especially for a portable device. In case of theft you don't have to worry about someone getting access to your files. Even if you don't carry around the machine there's still the possibility of theft from your home.
There're basically no performance penalties noticeable. This would only matter if you do something like live video editing of 4K video or something. But in normal operation it's not noticeable. And the process of first encryption has become really user friendly by making it on the fly, meaning you can continue work on your machine while it encrypts everything in the background.
As for Time Machine, you should additionally encrypt your Time Machine backups to be fully safe in the home theft case. You do that from the TimeMachine options for your backup drive where you'll find a checkbox labeled "Encrypt Time Machine Backup".