GTMetrix is another tool which analyzes your web page. You can use it on the following link:
Below are some of the reported points by GTMetrix and how you can optimize them.
Specify Image Dimensions
This point can be resolved by enabling the setting Add width/height attributes to images in Nitro > Cache systems > Page cache. Page cache also needs to be enabled.
If you have already enabled the above setting and GTMetrix is still reporting this point, it means that there are hard-coded images in your theme, which cannot be handled by NitroPack. You will recognize these images, if they are not located in /image/cache. Such images need to be manually optimized with an intervention to your OpenCart theme.
Combine images using CSS sprites
This should be optimized with a manual intervention to your theme, since it requires custom CSS code and manually combining images into sprites. This is a point, which cannot be optimized by NitroPack.
Use a Content Delivery Network (CDN)
This should be optimized with a CDN service. NitroPack allows you to sync your server resources with Amazon, Rackspace or a custom CDN via FTP. More information can be found in the CDN section of this documentation.
Use cookie-free domains
To resolve this point, please go to NitroPack > Image Optimization and enable the setting Remove Image Cookies.
Make fewer HTTP requests
Reduce the number of DOM elements
This point depends mostly on your theme and what modules you have enabled for the front page. NitroPack cannot optimize this point. In order to achieve a higher score, consider reducing the content, which you display to your customers, or switching to a more lightweight theme.