Consider too Kyrina that it might not only be a slower/older/smaller computer, but other factors too such as the type of harddrive connection, the type of video codec, framerate, resolution, etc. All variables that can have an impact on render times, playback smoothness, etc. For example, depending on the age of your computer, a harddrive connected at USB 2.0 speeds would be very painful to edit with.
With regards to rendering a project during the edit, it is generally individual preference, but turning off background rendering can help free up computer resources and minimize impact on harddrive space. But if a project is pre-rendered prior to export it can help speed up the export, if you’re exporting to the project rendering codec (usually ProRes). If you’re exporting an .mp4, that can take a lot longer to do since it has re-render everything. Generally, it’s a good idea to export a high quality master (e.g., ProRes), and then use another program like Compressor to make smaller, compressed versions for distribution, such as creating an .mp4.
Maybe all of this you already know, but figured I’d throw it out there since you mentioned still learning about all the puzzle pieces that have to come together for an edit (and editor) to work well.
Joby.