If you need to get troops out quicker in a pinch, slide the slider bar down when training troops. You will produce fewer, but they come out quicker.
I believe they are trying to increase the number of troops per hr (or per day) that the building can produce. Reducing the amount of units has no impact on that.
@Matrickz Victory springs is the only option. Simia Sapiens and brown bear help but because they work on all buildings your better to still use a different building then one that is 3x longer. Depending on your chapter the alternating increase in tech for training grounds can impact this compared to other buildings. If your on a chapter where the other building get their upgrade then the gap can be most painful.
I used training grounds initially when it came out, but like you ended up with it taking 3-4 times longer for many chapters, so it rarely got used. Managed to do well enough without those troops as qty of other troops outstripped the loss of quality by not having those specialists. Now I have the VS level getting up there it is better, except for elvenar chapter where it falls behind due to tech tree.