Does my toilet drain poorly because of bad venting?
It's a common misconception, but blocked vents do not cause slow drains. If you Google it, you'll find lots of incorrect information that completely ignore the physical realities of how a vent operates. The purpose of a vent is to prevent the P-trap from being siphoned, it is not intended to help water go down the drain any faster - in fact, a properly vented fixture  will actually drain somewhat slower than a fixture with no vent.
People often talk about a vent in the context of an inverted soda bottle analogy, likening the vent to putting a hole in the opposite side of the bottle so the liquid doesn't slowly &quot;glug&quot; out of the mouth of the bottle. That analogy is nonsense, because there is no such thing as a plumbing fixture with only one entrance/exit like a soda bottle - water going down your pipes does not need to &quot;switch places&quot; with the air ahead of it. Every plumbing fixture ever made has a separate inlet and outlet, and is already open to the air  at the inlet side, allowing the air to follow the water down the pipes. Adding or removing a vent does not change that at all - it is not analogous to adding a second hole to a soda bottle, as it's actually adding a third opening to the already two-ended plumbing system. A blocked vent could potentially contribute to slow drainage if the inlet side is also blocked and cannot let air in, but in that case, it's really the blockage in the drainage pipe that's the issue, which is only exacerbated by the blocked vent. I can't imagine a situation where water can go down the drain, but air cannot.
As an experiment, you can pour water into a sloped pipe and watch it drain as fast as gravity will carry it. Now add a T and a vent somewhere along the pipe, pour the water in again, and see that the vent does not slow the water at all. In fact, plumbing without a vent will drain slightly faster, as water accelerating through the pipes &quot;pulls&quot; the water behind it. Although a vent slowing the drain may seem counterintuitive, it makes sense when you consider that the purpose of the vent in the first place to to prevent the pipes from emptying completely! You can watch a video of such an experiment here - in a realistic setup with a drain, trap, and vent, blocking the vent actually makes the basin drain faster.
Any air that needs to be pulled into the pipe can be pulled in from the inlet - a vent would only help in draining a toilet, for example, if the lid forms an airtight seal around the bowl. The vent is just there to prevent a suction effect that would siphon the P-trap. Your venting may be done improperly, and it may cause different problems with your plumbing, but it is not the primary culprit of a slow drain. If anything, a blocked vent would result in a drain that is faster and smellier (due to the siphoned P-trap). Look for a blockage in the drain pipe, not the vent.