Hi.
The .sonar/cache folder contains only the .jar files corresponding to the plugins available in the SonarQube server. This means that, in the best case scenario, caching this folder will make the download of plugins a bit faster. It’s almost useless, as you can see.
There isn’t a way to speed up the analysis using caching. The SonarQube Scanner always scans all the files, there’s no option to scan only the changed files.