I think the key difference is the focused attention and brain bandwidth to spend. Traditional way is way more efficient but requires more of that - you need to do a relatively long sequence of actions, coordinated to the goal. Using voice, there is no sequence, and it doesn't matter how many times you need to try and to repeat the same simple phrase - you spend no additional bandwidth with every repeat.

The voice way is like writing the simple add-mupltiply loop in C, and UI way is like writing the same in assembly (using vector extensions). Assuming you know how to do it very well, writing assembly code for that loop wouldn't be even longer to do. And you don't have a 100% chance the compiler will do the right thing with unrolling and vectorizing in this particular case, but most of the time you check the output and try another compiler options until it do the right thing, and only after a bunch of tries you finally resort to assembly. That's basic energy-conserving optimization present in people's firmware in their brain.