I haven't owned a 3GS so I can't compare it to that, but I do not feel like my GS heater takes any longer or feels less strong than any other car I've had.

 

What's the outside ambient air temp?

Try pressing the "AUTO" button as well as the auto recirculation mode button to put it in fully automatic mode and then set the temperature to "HI" for both sides and see if this gets the cabin up to temp quicker. It'll direct air to the windshield and the floor while it heats up so it doesn't blast you with cold air.

Has the cabin air filter been replaced recently?

I would also ensure you're not in the Eco driving mode and if you go into your climate control menu, also ensure through here that Eco climate settings are not enabled.

Although I live in Los Angeles and we don't see the temps you might be used to seeing in MA, regardless, my GS has had the quickest heating system (from climate control system, to heated seats & heated steering wheel) out of any of our previous cars.

 

I used to have '11 GS350 AWD and I don't feel like the heater takes any longer to heat in my '15. We often go below 30degrees F here and sometime single digit(like 2 days ago) and I'm not afraid that any of my vehicles not having adequate heat.
However, I do feel the '15 AC is not as cool as my '11 was. I mentioned it in one of my service at the dealer and they said to crank it up.

 

 

You're not in Eco mode are you? Make sure to be in Auto with the AC on (not off!) for the vehicle to do its thing. If it's still super slow, then get it checked out.

 

I don't get heat out of the vents until I set the AC to 84 fahrenheits, which I thought was weird.

 

I have no issues with the climate control. I have it on full auto and leave it at 74F...it does the rest fairly quickly and efficiently.

 

I leave my heat set between 70-72. System is on full auto. Car is not in eco mode. I feel like it heats up at a normal speed, probably faster than the past two cars I've owned. I have noticed that it won't shoot much air out of the dash vents until the air is actually warmed up. Someone else in the thread said the air comes out the defroster vents and foot vents until it is warm, so that you don't have cold air blowing on you, that seems accurate based on my experience. Maybe that is why you are getting weak air?

Also, I use the S-Flow setting. That is supposed to heat only the area where there are occupants. I believe in my car (luxury package) there are occupancy sensors for driver and passenger. If I have people in the back seat I need to manually disable S-Flow. I am not sure if non-lux package cars have a passenger occupancy sensor, I think they may not.

 

Non-luxury packaged cars have a more basic system. I don't think it uses sensors in the interior like yours does but looks at whether or not the doors have been opened, something along those lines. In my GS F-Sport without the more advanced version, when S-Flow is on, it controls air to the rear seats as well as the heated/cooled passenger seat. When enabled, if the passenger seat heater/cooler was previously left on, when I start the car, I see it turn on for a second but then automatically turns off when it realizes there is no passenger. It does not, however, turn off the passenger side temp control like I think happens in yours, but frankly I don't really know what that actually does on the more advanced versions.

When that happens, is it also disabling the passenger side air vents? Or?

 

I do think that my passenger side temp control turns on for a second before the car realizes there is no one there, then it turns off. From what I understand/remember, S-Flow will use all of the vents in the car at first, if necessary, to get it to the right temperature, then I think it usually limits itself to the driver's side vents, or driver's and passenger's if there is a passenger, as long as that can maintain the desired temperature level for those zones. So yes, I think it can totally turn off the vents for passenger and rear.