Slow drainage from garbage disposal

I apologize ahead of time if this issue has been answered but I looked and couldn't find it. Anyway, the problem is, when I have the non-garbage disposal side holding water, stopper in, and then try to use the garbage disposal (GD), it will not drain or is VERY slow to start and drain. There is not a sudden and powerful suction on the GD side but as soon as I remove the stopper, it drains. My theory was a vent or vacuum issue but as you can see, it has a REDI-VENT. I took the drain, leading from the GD to the other sink, out along with the trap. There were NO clogs/ stoppages, plus water drains fine except in above scenario. Please help.

try it again but loosen the vent and see it it starts draining

When the disposal fills with water and fills the horizontal pipe, there is no where for the air to go that is being displaced by the water. With the drain not closed off, the pipes fill fast pushing the air out and then the movement of the water without air to be displaced is much faster. I don't know of a solution - I have a single sink and I've never liked how slow it is when the sink is half full and the stopper is removed. But in your case it sounds like it barely drains.