0% is only slower when converting from a wide FOV to a narrow FOV, like from hipfire to a scope. When converting the other way, from scope to hipfire, it's opposite. 100% is slow and 0% is fast. This is obvious if you think about, since a reverse calculation should always work.
And a 2D conversion like from Windows is essentially an infinitely small FOV, so 0% will always be faster than 100%.