We probably would have sacrificed less had we concentrated on defeating Japan first while letting the Nazis and Soviets slaughter each other - then we could have hit the weakened Germans. In effect, the Soviets did this when the stayed out of the Pacific war until we effectively defeated Japans; then they attacked the retreating Japanese in Manchuria and Korea.
I doubt if there is any way the US could have defeated Japan faster than we did, even had we ignored the European theater to focus all attention on Japan.
Remember, among other things: that effort required a huge Navy with over 100 aircraft carriers, large & small, and many hundreds more of other war and transport ships.
These were still being produced in 1945, and no way would the US have invaded Japan without them.
And in the end, the US did not invade Japan, instead our A-bombs finally convinced the Emperor the fight was over.
So it simply is not possible for the US to have defeated Japan before August 1945.
And had Germany still been in the fight then, the US would have turned our A-bombs on Germans -- since they are who the Bomb was originally developed for.
Also remember, history records several attempts by both Stalin and Hitler to reach a separate peace.
Thankfully none were successful.
In 1941 Hitler brushed aside Stalin's feelers, and after 1942, when Stalin was convinced of his ultimate victory, he would not entertain any German peace feelers.
But Stalin constantly demanded Britain and the US open up new fronts in Europe, to take some pressure off his armies.
And Churchill and Roosevelt constantly promised Stalin new fronts, beginning in 1942 (North Africa) and every year after-wards.
Had Stalin become convinced the US and Britain would not come to Russia's rescue, he doubtless would have done just what those same Communists did do in the First World War -- sign a separate peace.
Bottom line: all things considered, in a war where something like 75 million people died, the US achieved the greatest victory with the least loss of US lives that anyone could realistically imagine.
So any proposed alternate scenarios have a major burden of proof to show how they would achieve the same victory with even fewer US military casualties.