The fly line speed cannot ignore the space used to cast. In other words, the more space you have to carry out the entire cast and the more acceleration you can produce.
This will yield a faster line speed. Casting with light lines, you have a small mass to cast and the space in which your cast takes place becomes important and so a vertical over the shoulder cast can limit your results.
So going down to thirty or forty five degrees increases the space in which you are casting. This is because the way we are made physically prevents us from using the whole casting space to a maximum and so going down to thirty or 45 degrees increases this space considerably. We should not forget that an inclined plane of casting can create problems with respect to the relationship with the axis. This is because the force of gravity can and will have more of an effect on the line.