Install the Supplemental Update
Fix addresses a software bug in the firmware that affects processor speed. Roman Loyola shows how to install it
First, perform a backup of your Mac before installing the update. Your Mac will also need to restart.
1. Launch the App Store app in the Applications folder, or click on the in the menu bar, select About This Mac, and then in the Overview section, click on the Software Update button.
2. Click on Updates in the top bar of the App Store app.
3. Look for the macOS High Sierra 10.13.6 Supplemental Update in the listing. If you don’t see it, go to the menu bar and select Store > Reload Page or press Command-R to refresh the Updates.
4. Click on the Update button to the right of the Supplemental Update listing. The update will download and the installation should start automatically. Your Mac will need to restart.