Why is HDD speed related to ReadyBoost?

The point of swapping the page file onto another partition (for me) is the that file remains contiguous and is therefore written and read fast. Windows usually creates the file as a contiguous entity - assuming that there is enough free contiguous space.

Windows tries to grow the paging file dynamically. This means that the growing and shrinking paging file gets fragmented rapidly and starts decreasing overall performance.

For the above reasons I've found it better to set the page file to a fixed size.

I'd agree that a better performance boost can be had by swapping the page file onto another drive! However I've not needed to do this. I did consider swapping the page file onto Virtual RAM but for system stability reasons I decided against doing this.

As for loading programs - Virtual RAM does essentially the same job as superfetch and so superfetch isn't needed. It actually hinders performance if left enabled on my machine.

If the initial size of the pagefile is adequate then it will NEVER resize. There will be no continuous resizing, even if the pagefile is system managed. The pagefile will only be resized if this is necessary to prevent exceeding the commit limit. I have confirmed this by observation many times.