Hi @marcorinia,
Great question! The Performance Lab plugin is indeed used to test experimental features, many of which will migrate from the plugin to WordPress core. The plugin itself won’t be merged, it’s more than many of the features or performance enhancements that are achieved through this plugin will move to be part of core WordPress. This has already been happening, and will continue to happen! Some health checks (ie. Persistent Object Cache status) and improvements such as Fetchpriority have already been merged.
Let me know if you have any questions with the above. You can also keep tabs on what’s coming to the plugin, or what’s being merged over on the plugins GitHub repository, or taking party in the weekly chats.