I have an old brick home that I have redone the bathroom in a couple of years ago. The bathroom sink has always drained very slow. When I redid the bathroom I put in all new plumbing in the bathroom so I thought this would fix the very slow drain issue. The pipe for the bathroom sink goes straight into the main stack. I don't have a problem with any other drain not draining in the house. The water could sit in the sink for hours but will eventually drain. I have taken it apart and snaked it and drain cleaner I have done it all. My question is would adding a vent to the roof help in this situation to allow the vacuum I believe that is causing the slow drain correct this problem? Thank you for any advice in advance.

Does that main stack go up as well as down after this sink connection? If it does its likely you vent stack and this could be a wet vent. Have you ever been on the roof, is there a vent stack sticking out of the roof?