Tools within a workflow needs to be able to run in parallel whereever applicable.
For example: Extracting 10 million rows from one source, 12 million rows from a different source to perform blending.
currently the order of execution is the order in which tools are dragged into the canvas. Hence Source1 first, Source2 second and then the JOIN.
Here Source1 & Source2 are completely independent, hence can be run in parallel. Thus saving the workflow execution time.
Execution time is quite crucial when you have tight data loading window.
Hopefully alteryx considers this in the next release!