In an effort to speed up operations within the ConfigMgr distribution component, you may not want specific file types to go through the compression function of ConfigMgr. When content is distributed between sites in the ConfigMgr hierarchy, ConfigMgr will go through a process to compress all files except for those file types listed in a specific registry key. You can access the registry key on the site server to get the full list and even add file extensions that you believe would speed up the distribution process.
Data type: REG_SZ (string)
By default, it contains the following extensions: ".cab;.zip;.arj;.rar;.jpg;.jpeg;.wma;.wmv;.mpg;.mpeg;.mov"
One specific example, as a best practice, that you should definitely add is the .wim extension for operating system deployment. WIM files are already compressed and further compression is not going to make any difference, but only cause a delay in how long it takes the package to hit the distribution points.
Of course, it’s up to you to customize what makes sense. Test in a lab environment first (as always) to determine which file types are best excluded from the compression process.