Etermal

Etermal is an embedded terminal for OpenGL 3.3 that enables you to extend the functionality of the interface by customizing or defining your own shell and terminal implementations. You can get started by including the Include folder contained within it in the Include Path and that the Library binary is in the linker path.
According to the developer, the library contains a frontend and backend, which act as a terminal and respectively the shell. As you probably hinted, the terminal is in charge of displaying text and your inputs, whereas the shell enables the typing and processing of commands.
On a side note, the shell includes an argument parser which is not exactly very impressive. This is why you are encouraged to make your own custom one. As far as the frontend terminal is concerned, it comes with several advantages, including but not limited to colored text, scrolling, UTF-8 support and dynamic initialization of OpenGL resources.
Given the nature of the utility, it is only natural that you question the performance. In terms of CPU performance, the developer claims that benchmarks show that on a rather old laptop the FPS is 2000 and higher. In terms of memory, the footprint is rather small, so it should not be an issue.