Hi! I am a new user in Unity. I started using Unity like 2 months ago. I want to say that it’s a major drawback for users like me who have very low-end PCs to run unity smoothly. Performance could be improved greatly if Unity makes a built-in C# script editor. It would be easy to have a tab in unity, where we can write code, without using external tools, like Microsoft Visual Studio, Visual Studio Code, JetBrains Rider, or Monodevelop.
No, that wouldn’t really help with performance. The question is how many features do you expect from your code editor? VS does an incredible job in supporting you for a “relatively” small footprint. If you want a fast ultra low footprint editor with no features, use notepad 
You can use any editor your like but you would of course loose the highly complex tools that the IDE brings with it.
Unity used to have a simple script editor called “UniSciTE” which specifically supported the now outdated UnityScript. It was a small editor comparable to notepad++. Though it didn’t have the feature set nor the stability compared to VS.
There are simple script editors on the assetstore that add an editor window with a simple script editor. Though do not expect much from it. Keep in mind that a proper IDE also provides a debugger to live debug your project.