My experience with Volvo racks leads me to believe that you are speaking of a Thule product (That's branded by Volvo). And the part that Thule doesn't do so well is the need to tighten the towers down every time the racks are mounted, and the need to loosen the towers every time the rack is removed. Yes, you can do it by hand, but the fact that it needs to be done every time is a problem for me. I much prefer a system that requires ONE initial adjustment (with or without a tool) and then works by throwing a lever/cam for mounting and unmounting from then on. I'm talking about a 40 second process each time instead of a 5-minute process each time.Our Volvo XC90 bars with T-Slot don't need any tool. They have thumbwheel wing nuts to tighten and loosen. But they are locked to the car with keys that I keep in the glove compartment. For one person, they take about 4-5 mins in total for both bars to install / take off. It will be slightly faster with two people.
The 3 has built-in receivers for the tongue of the rack feet. There's no indication that this system will be more "permanent" than what you've experienced with the Volvo. Of course "easy" and "quick" are both so wildly subjective as to be meaningless.However, the Volvo has built-in rails to mount the bars. Model 3 doesn't. That's kind'a why I'm wondering if the mounting is going to be more permanent. Tesla's copy says "easy installation at home", not "quick installation". I hope it is a quick on/off thing.
So we wait and hope.