What is the sending speed with Pro?
The sending speed depends only on your internet connection.
On Smash Pro, transfers have priority treatment on our servers, contrary to the free version (transfers bigger than 2Gb don't have priority treatment).
Concretely, this means that once the loading is finished, they are available for download immediately. There is no waiting time, nor any delay between sending and receiving by the recipient.
That's why we can say your transfers made with Smash Pro are faster. But the speed of loading and therefore uploading is only linked to your internet connection.