They’re not trying to go fast. They’re trying to get to the target altitude to test the flip. No need to rush or overshoot.

In “production” it will be atop the booster, so taking off like this from sea level isn’t part of the plan.

They don't want to go to high or to fast. So they start out pretty heavy and as the fuel is consumed they turn of engines in order keep the TWR the same.

They don't want to test supersonic flight profiles with this vehicle.

This flight profile is not at all optimized for fuel use.

Each Raptor engine cannot throttle down below about 40% of its maximum thrust [0], so they must completely shut down some of the engines in order to achieve the lower required thrust for hovering or landing.

[0]: https://twitter.com/elonmusk/status/1295553672454311941

It is less efficient. That said, they aren't carrying a payload and they don't have to run it full on fuel, so it doesn't matter. More importantly, they care about testing the landing which requires them to be a certain horizontal distance from the landing site, so it's easier to do that by hovering sideways so they don't get way too much height.