The first thing would be to check if your vacuum is good at the storage tank that'* under the right front fender (assuming it'* the same on your Bonneville as a leSabre). The vacuum comes off a small line next to the brake booster on the intake manifold. It goes to a Y connector by the AC drier in front of the passenger and one side goes inside the car and another stub line goes to the storage tank. See if the tank is cracked or that it stores a good vacuum after motor is shut off for several seconds when you pull off the line there.

Then go inside the car and see if you have vacuum at the violet line from the programmer. It connects to the black 1/4 inch plastic line from the engine compartment right up against the A pillar. It does pull apart if you work on it.

Because you say several mode changes are slow, I'd suspect low vacuum rather than just a problem with the nipples inside the plastic connect in the imcool.com link. There it'* usually the dash vent line that gives problems first and other switching operates normally, i.e., floor, then switch to defrost, then back to bilevel.

You can patch the lines if the nipples are the problem with aquarium line that slides over the vacuum lines or I used 1/8 inch gas line, free from the local parts store'* junk bin.

The only switch that should be slow is when you turn on or off the recirculate; it has a porous plug in the line to slow down the action.

You might have a leak in a diaphragm in one of the vacuum motors, so you might want to look above the accelerator area and play with them while operating the AC controls for switching. You can move most of them with your finger on the backside from the vacuum connection.

Defrost is the default with no vacuum. That'* why it'* there when you start the car. The floor vent closes, the dash vents are closed and the air comes out the defroster. It take vacuum to effect any of the other settings.

I checked the motors by switching a vacuum line that was operating in a setting to another vacuum motor temporarily to be sure that motor worked.