I also have a late 2009 Mini with HDD (7200 rpm), 8GB RAM attached to a real monitor running Snow Leopard. The 2009 Mini uses wifi. When screen sharing to the 2009 from my 2012 Mini (El Capitan), the response is good. I haven't done much screen sharing to the 2009 Mini but I would think this would be typical. I noticed that Screen Sharing doesn't take much memory at all so a RAM upgrade won't necessarily help if Screen Sharing is the culprit in your case. But as g33k points out, a RAM upgrade would help the other applications you're running and boost the overall system responsiveness.