With fast weight loss comes inevitable muscle loss.
And loose skin
Yes it's better although not in a linear way. At a certain rate of weight loss (dependent on genetics, current body fat, etc.), the ratio of fat vs. muscle loss will start to get worse.
I always read .5kg/1lbs a week is best...seems so little.
IMO the best indicator of muscle loss is loss of strength. So as long as stength in the gym is OK, a greater rate of weight loss should be no problem.
In my experience that is a hard thing to measure unless over an extended period of time. I mean, with mean, I have super off days when I running on 2 hours of sleep or have pains in my back or whatever. I guess you would have to keep track over a long period of time. Also, I never try to set records when on a diet so it is also hard to tell for me.
I've lost around 20 pounds so far and it took me around 4 months.I improved on all of my lifts, though I have never been strong.If a person is around it's maximum strength and muscle wise, it may be a very different situation.
As long as protein requirements are met and you are inolved in a resistance program it doesn't matter how fast you lose.