the bigger the front the slower you harvest

While I fully understand that in the game you might want to harvest a bit faster, the reality is that the game is already not in line with reality in terms of harvesting speed. It all depends on the conditions of the field, the crops, the time of day and the size of the Header. If your field is littered with weed then harvesting tends to be considerably slower than when you have a "clean" field. Also, when you have a good harvest you tend to go a little slower than when your fields bear very little fruit. In real life, if you can harvest at 8KPH you are going quite quickly. If conditions force you to go at around 4KPH then it's considered tough going. Just talking from experience... something GIANTs might want to consider in one of their future FS Games?