for large lists it is recommended to use folders.
Although folders (also called containers) are not required to use large lists and libraries, you can still use them to help organize your data and improve the efficiency of your data access. When you create a folder, behind the scenes you are creating an internal index. This internal index is also created for the root folder, or top-level of a list or library. When you access items in a folder, you are effectively using this internal index to access the data
how can I know all default internal indexes? is the content type an internal index?