Generally speaking structures are faster than classes, but that difference mostly comes in play in certain situations. MSDN has a nice article about the differences: [LINK]
FlowCanvas can work with both classes and structures the same and the most commonly performance issue with structures (value boxing), is not happening in FlowCanvas either, except in the case of when data “travel” in and out of a Macro, in which case a structure will be boxed/unboxed.
Gavalakis
