Hello, 

Speed is a skill underlined for GK. But when I increase it, my GK's averaque quality doesn't increase. Is speed still useful for a GK ? 

From my point of view it doesn't work. You said it yourself: the quality of the GK does not increase so that is enough.

Speed is not a quality that gk use. Instead, train the strenght, it will increase the avq of gk

Well, I think speed makes an important role in real life for a goalkeeper.. surely the strength is another important factor beacause GK are explosive… but I think it will do something for your player