Please allow the auto slipstream feature of SETUP to store and reuse extracted updates. Currently, SETUP can download and extract updates every time it is used. When an UpdateSource folder is provided, the SETUP will not need to download the updates, but it will still extract the update each time it is used. With a network install used by many developers, the extract of the same update is done many times needlessly.
I'm not referring to saving the original update (e.g., SQLServer2012-KB2723749-x64.exe). The extracted update can be manually created via /x: option. For example,
SQLServer2012-KB2723749-x64.exe /quiet /x:"path"