Let me go with this step by step, We do need a C++ runtime for building to native. It should be installed automatically when you install the engine or already there when you installed VSC or other programming work. You can see how to export to native platforms here.
https://docs.cocos.com/creator/manual/en/editor/publish/native-options.html
Cocos Runtime is a different product for large app developers that allows you to add an HTML5-wrapper into your app so that you can add a gaming area to your app that improves the playability of these games. Mostly used for WeChat, Taobao, and other apps. This is an option for export in the engine, but currently not many options for western developers who do not want to bring games to China.
As of the memory usage, I'll check with the team on this and give them feedback. Thank you for your message, we take these issues seriously and work to improve the engine with every iteration.