Installed new walk in tub. Takes approx 41/2 minutes to drain. Supposed to drain in 2 minutes. 1-1/2 inch drain in tub, connected to new 2 inch pvc pipe which is about 3 feet long. This connects to 4 inch stack (?). If I change the 2 inch pipe to 1-1/2 inch will that create more suction and drain the tub faster? We tried a home made studor type vent, but that didn't seem to help.

Walk in tub! Is that one of those critters with a door? Does it have a overflow like regular tubs do? Tell me where you installed the AAV?

If I change the 2 inch pipe to 1-1/2 inch will that create more suction and drain the tub faster? Downsizing the drain line won't help.
Back to you, Tom

Yes, it is a tub with a door. We tried the vent in the line between the trap and the stack. If a smaller line won't work, what about a 3 inch line to the 4 inch stack? The 3 in. line with a 3 inch fall is supposed to drain 35 gallons/minute, but the drain in the tub is 1.5 in. going into currently a new pvc 2 in line.

You're stuck with what you have. A 1 1/2" drain will only allow so much volume to pass. It doesn't make any difference if the drain discharges into a three inch pipe or a four foot storm sewer it will still drain at the same rate depending on the head pressure. Either way increasing the drain line won't let it drain any faster.
Please answer my question, "Does it have a overflow like regular tubs do?" If so, does it gurgle when it drains? What are you venting the tub with now?
Sounds more like a vent problem to me. Regards, Tom