Internal storage management of files and folders My Xiaomi phone stores some folders inside internal storage (cannot store app data to external) after installing some apps. And, when that app is uninstalled, these folders remain there. Also, I can see several files ( may be system files) with and without extension other than app folders. Cleaner application also not clearing folders and files.


Why these folders are not automatically get removed, during uninstall?
Whether files,that have no extension and with extension, creates slow down of device?
Without a list of your internal storage content, it's hard to discern useless files from useful ones. Without further info, the only answers I can give you are the following:


When you uninstall an app, the system deletes the trees corresponding to the app's package name under /data/app/ (application executable) and /data/data/ (application's internal data). It should also wipe some folders into your internal and external storages, namely those corresponding to the app's package name, and placed under Android/data/. Any other folder, created by the app in a path different from the abovementioned gets ignored.
It may depend. Generally, those which slow down badly your device are files which are big in size. Since, as already said, they're not cleaned by the system, that's a burden that falls on you, the user.


As a footnote, the files residing in an internal storage are not fundamental (and I remark that I'm talking about files, not folders). Generally, they can be files which are placed there by an app for the purpose of flag, meaning that said app looks if such file is present, and performs something. Else, it does something else.

Finally, if you delete any of these files, they may be regenerated, if the app that created them is still installed.