You're overrating the importance of processing speed and underrating the importance of Dickinson's tools. Yes, in order to be a Makar/Hughes/Heiskanen franchise level defender you need elite processing speed and elite tools. Dickinson is likely never going to reach that level and neither is Buium.
But there are plenty of big top pairing defensemen like Jones, Ekholm, Chychrun, Nurse, Sanheim, Parayko, Gavrikov, Ekblad, Harley, etc. who are not the quickest decision makers but still manage to play 22+ minutes a night with solid to great results. They know their limitations and play a simple defensive game of snuffing out rushes before they happen, winning board and net-front battles with their size and blocking shots while occasionally taking over shifts once the puck is in the offensive zone. I understand that some of those elements of the game are still a work in progress for Dickinson but I'm fairly confident he can develop into this type of top pairing dman which is why I wouldn't even consider trading him for a pick that would get us Hagens or Martone.