Impact of maven.artifact.threads

From what I see, the default number of threads when downloading artifacts in maven is 5.

I have tried to increase it in my project to 10 and I do not see much difference.
(It takes ~30 minutes of downloading for a clean build, CPU has 16 cores)

Is there any documentation which shows the impact of increasing that value on total artifacts downloading time?

Is there a way to make maven display in logs which thread is downloading which artifact?