Have a place in the deck's settings where one can enter a LaTeX preamble; that is, text that gets inserted automatically at the beginning of each LaTeX snippet prior to its compilation. This would be a useful place to put \newcommands so that they are available to all math snippets in the deck.
amanda wallers commented
Tex Studio's command prompt function works very slowly to display text to the screen (delayed due to running suggestions), how can I turn off this feature of Studio?
This is the reason I am reluctant to use slides again. I like it for the integration of latex and easy image insertion and wysiwyg layouting for putting slides together fast. But the fact I can't put a preamble of shortcuts like \newcommand globally for a presentation destroys so much of the speed advantage that its not really worth it for me.