If you mean the installation when you say "add this to your browser", then this is normal. The extension contains the libraries which are otherwise downloaded from the internet and this is why it is 20 MB in size. So far no one has reported any performance problems and none of my own installations have such behavior, but I only use Firefox and LocalCDN on different Linux distributions and Androids.
To analyze this in detail further information (operating system, hardware, other extensions, etc.) and tests are required, but I cannot do this because it is not reproducible on my side. Try it with a fresh new profile and only LocalCDN. Maybe the problem is caused by the combination with other extensions. You can send me the results by e-mail.