Why bother installing a custom ROM?
We are all familiar with the time where your phone decides to play up and refuses to do what you want it to do. Anecdotal evidence has found that all smartphones degrade over time due to heat, stress, wear and etc. However this doesn't mean you can't still make improvements on your phone.
One common cause of slowdowns on your smartphone is the presence of bloatware or "bloat". These apps are unneeded and unwanted, meaning that they take up your important storage space and memory. A custom ROM however never contains any bloatware. This will mean that you will be able to reclaim any of the lost storage space and memory and use it for more important applications. This will translate to better performance (and sometimes battery life).
Custom ROM options
LineageOS isn't the only custom ROM available. There are heaps of other ROMs you can choose from (Resurrection Remix, CR Droid, Paranoid Android, etc.), each coming with their own unique combination of features and apps. To get started, it is best to consult on a thread over at the XDA-developers forums (Wikipedia). XDA is the first address of all your Android needs.
Like other desktop-operating systems (Arch, Debian, Mint, Ubuntu etc.) you have to find out for yourself which ROM suits you best.
LineageOS is an open-source operating system (OS) for your Android device. In the development community, it is often referred to as a custom ROM (or sometimes aftermarket firmware). Android is generally open-source (AOSP - Android Open Source Project), however the Google Apps (GApps) or other manufacturer (Samsung, HTC, ...) applications aren't.
LineageOS has their own option to get root access on your phone. Just flash the zip through TWRP. (later more)
Resurrection Remix OS is based on LineageOS with much more settings / features.
> If you want you can add more ROMs. Got to PwOSS - Gitea