alt Hacker News> the second of which is a linear interpolation of the frame before and after it
If I understand what you describe, this is generating a frame "in the past", an average between 2 frames you already generated, so not very useful? If you already have frames #1 and #2, you want to guess frame #3, not generate frame #1.5.
The higher the "real frame" rate, the smaller the differences from one to the next. This makes it easier to predict those differences, and "hide" a bad prediction. On the other hand if you have 10FPS you have to "guess" 100ms worth of changes to the frame which is a lot to guess or hide if the algorithm gets it wrong.
I chose the two scenarios I did to illustrate that "frames per second" is clearly not meant to be measured in terms of times the display refreshed, but rather in terms of times content was actually rendered by the game engine.
In my opinion it is quite difficult to provide a definition of "fps" that somehow makes 45-fps-native-with-frame-doubling be counted as 90 but doesn't also make either of the ludicrous examples I presented be counted as 90.