How can I fix a very slow drain?
Clearly the restriction is left of the sink in your diagram.  One more question you could consider is whether this is an installation deficiency (problem has always existed) or whether the drain was working properly at one time and the situation has become worse.

I would suggest using a video inspection camera to be certain about the location of the restriction and perhaps what is causing the restriction.  Here is a rental unit that may be available in your area.  Or here is a contractor that could provide this service.  It would be best to try to clear debris before the blockage prior to a camera inspection.