I am amazed how, if a background job has been active, I have a hard time logging in because Ubuntu drops my keystrokes on my USB keyboard. You would think that support for interactive IO would be in hard wired kernel or supporting app pages.
Similarly, when I click, especially in Chrome, often the system has the wrong or dated location for the mouse, and I get the wrong window or other selection. You would think that mouse activities would be buffered so that when I click the app knows the mouse location.
The amount of background activity, the number of pages not in RAM, should not affect interactive inputs.