I was really near to move to Simplify3D from FlashForge, but I saw that in Simplify3D absent one killer feature of FlashForge. Yes, Simplify3D has option to combine infill layers. But in FlashForge this do better. Look:
FlashForge has 3 options for this: how many it may combine solid infill layers, how many it may combine spare infill layers and threshold when do not combine to prevent overheating of a layer.
First two option is convenient. Will may set perimeter 0.08, combine solid infill by 2 and get 0.16, combine spare (triangle or honeycomb) infill by 4 and get 0.32. So I will get very fine surface with relatively quick printing. Of cause the first thick layer must not be combined with others (and must be defined in absolute height, as already said on other feature request) and may be other exclusion must be too. For instance bridges must have own thickness settings and it must not combine with infill.
Very important threshold setting, the Simplify3D lack of this and so combine option in Simplify3D is near useless. Because on some layers only perimeter will be printed (this rather quick) and in case of small details a layer will be overheated. So threshold define when do not combine. In FlashForge this setting none wise defined as area of the layer. But much better if threshold will be defined as time to print layer, in this case to print perimeter. If perimeter is printed quickly, for instance less than 2 second, turn off combining infill layers.
Re: improving of combining infill layers
Thanks for your feedback! The combine infill every X layers option is taken into account when calculating layer times for the automated slow down for layers below a X seconds feature in the Speeds tab. The combining of the infill is not stopped but the layer is automatically slowed down to print at a slower speed if the layer time calculated is below the set threshold.
"A bird does not sing because it has an answer. It sings because it has a song."
Re: improving of combining infill layers
You can not very slow down. 
There is a some limit. For instance, in my experiments with Creator 3 and PETG I found that printing with speed below 6 mm/s lead to loosing of quality (overheat). So, if you will need to make more slow down, you will have to make pause or... type infill.
There is a some limit. For instance, in my experiments with Creator 3 and PETG I found that printing with speed below 6 mm/s lead to loosing of quality (overheat). So, if you will need to make more slow down, you will have to make pause or... type infill.