The key here is (probably) the project settings. They are normally established automatically when the first clip is put on the timeline. So, potentially, if you selected QFHD at the start, it may have changed unless you checked the "Force Selected...." box. It is important to put a "primary" video clip on the timeline first, moving as necessary later.
Further, what you see during editing is a real time preview. Depending on your hardware, project settings and media it may not be as good as you would like. At final output, the computer will take it's time to render a better product.
If there is a yellow line above the timeline it is a single that you should try to help the computer by "rendering" a fresh preview. Press "Enter" to do that. It also may mean that the project settings are wrong.