Actually, it isn't the Japanese at all. It's everyone else in their field of play. WWII happened in a world that didn't really catch on to what was happening until it was in full swing. And then everyone exploded. Japan, Germany, Italy, and USSR were abel to gain what conquests they did because of this situation. The Axis or the Commitern could have won complete victory if they had been able to do a little more than they did. By luck the democracies of the world woke up at the last minute to find the theif in their backyard.
But in this game, that element of world doggidness does not fully exsist. Oh, sure there are restrictions to more democratic nations, but instead of alot of nations fighting for survival, it's as if all of them are bent on conquest. So now you have a Japan amidst a sea of enemies bent on conquest which was not the case during WWII. This means early in the war, Japan ends up on the defensive, primed and ready for alot of American invasions later on. My last game ended with most of the east of Asia all American blue.
I always rewt for Japan, and I wanted to see them do better as my Axis partner (I never include Italy, I want all their lands for Deutchland!). So I made several adjustments. Namely, I took alot of pressure off of them. Japan can handle most of it's neighbors, it's the combo of USA and USSR that give it troubles in the long run. Also Nationalist China can get a bit greedy as well. I kept seeking peace with them and kept Japan out of trouble with the Chinese lands it held. I took a quick decisive victory against USSR and forced a bitter peace giving Japan a sizeable amount of landmass in eastern Asia. With all my attentions on USSR, I did not begin an Afrika theatre until 1946. I also suffered two Normandy-sized invasions (both of them actually on the Normandy coast) which left a good half of my forces in a near shambles. Once bitter peace with USSR, I was able to refocus and finally kick the rest out of Europe (R-Spain, N-Spain, Portugal, Italy, Vichy). It took my first nuclear bomb to dislodge the Brits from Gibraltar. An expensive way to go, but it is a necessary port to own.
Japan was doing well but, USA was creeping up. I began mass producing ICBMs. I figured a good way to help Japan was stall the USA. Thus began the terror bombing of continental USA, and it has had a profound effect on their war contributions. I like to group 4 ICBMs together and hit one American province with each group of 4. A strategic strike with this method does alot of damage. Anyway, the USA stopped advancing. you could tell they were balancing economy and trying to recover, but I kept the missles coming. Also, I sent my last of two nuclear bombs to D.C. It's 1947 and I am anxious to see how Japan does this year. At least they knocked the USA out of Iwo Jima already.
One thing I will say about Japan, they do seem a bit slow when it comes to reclaiming territory that is right under their noses.