I do not understand "the calculation does not add time to a transfer". I understand it does not slow down downloading but it clearly adds time to the over all process as its spending time calculating instead of going straight into downloading. It appears that about 5 minutes is spent "calculating" when it could just go into downloading. To me, that is time wasted calculating.
No it's not. Normally, WinSCP lists one folder, transfers the files, and then lists another folder, etc. With multiple connections, WinSCP needs a full list upfront. And then it does not list the folders again. So while it takes longer for the first transfer to start, the overall time of the whole operation is the same. Well, actually is shorter, as you are using multiple connections. So turning off multiple connection will definitely make the operation longer.