In regards specificly to the UFO test... The blurriness is generally not related to frame rate but LCD persistence. Ie, on a theoretical zero persistance 60hz display you would not likely notice the individual frames getting blurred from frame to frame. In this case the only real difference between a 60hz and 120hz display is that the 120hz display would have additional frames of motion. Your vision system would then has less frames to interpolate between.
In reality though, all LCD displays have persistence issues. (You end up seeing bits of the previous frame suspended over the current frame).
Higher quality monitors designed for higher display frequencies tend to also be tuned to have less persistence per frame. There are also tools/tricks some brands employ to remove / reduce the persistence (and blurring), which can be effective (or annoying, depending on how they do it) even at 60hz. There are also "120hz" and higher displays with such bad persistence issues that they are way worse off on the ufo test than a good low persistence 60hz display.
As an easy example. IPS panels tend to have a lot more trouble switching between frames quickly than VA or TN panels, so they also tend to exhibit more persistence issues per frame. This is then directly noticeable on the UFO test between these kinds of panels. IPS panels have of course been getting a lot better in this regard in recent years!
In reality though, all LCD displays have persistence issues. (You end up seeing bits of the previous frame suspended over the current frame).
Higher quality monitors designed for higher display frequencies tend to also be tuned to have less persistence per frame. There are also tools/tricks some brands employ to remove / reduce the persistence (and blurring), which can be effective (or annoying, depending on how they do it) even at 60hz. There are also "120hz" and higher displays with such bad persistence issues that they are way worse off on the ufo test than a good low persistence 60hz display.
As an easy example. IPS panels tend to have a lot more trouble switching between frames quickly than VA or TN panels, so they also tend to exhibit more persistence issues per frame. This is then directly noticeable on the UFO test between these kinds of panels. IPS panels have of course been getting a lot better in this regard in recent years!