Hello,
By default, the Background Image is resized according to the Slider's Height. You can consider using the CSS Code below to make the Slider's Height according to your needs to what suits your Image best:
.device-sm #slider { height: 400px !important; }
.device-xs #slider { height: 300px !important; }
This will definitely work fine. The Responsive Classes are as follows:
.device-xl – Class for Large Devices >=1200px
.device-lg – Class for Medium Devices >=992px to 1199px
.device-md – Class for Tablets
.device-sm – Class for Landscape Mobiles or Phablets.
.device-xs – Class for Portrait Mobiles
You can add these Custom CSS codes in the css/custom.css File. Additionally, make sure that you have checked out the Documentation > Start > Customization Section for complete details on how to use the css/custom.css File.
The Slow Loading issue is almost never an issue with the Template but the Amount and Size of the Media Files or the Server Configurations and Speed. Please consider checking out the Documentation > Start > Optimization Tips Section and consider implementing the tips provided as these should definitely help speeding up your website.
Hope this Helps!
Let us know if we can help you with anything else or if you find any further issues.