Some players temper their games because they think it's futile to crack the ball when the surface is going to slow it down
It is futile. Almost all ATP players do this. That's why the average first serve speed is slower at RG than at other slams.
The emphasis is more on the placement of the serve and less on the speed. It's the way I was taught to play on clay, (I still suck on the surface lol)
The surface negates the pace of your shot, giving your opponent just that bit more time to get to the ball. You will make more errors hitting harder, riskier shots but you won't benefit from putting pressure on your opponent because of the surface.
The gritty surface of clay causes more friction when the ball contacts the surface so it doesn't move through the court as much but the friction gives the ball more topspin than a regular hard court so it bounces higher. (I HATE playing on clay)
but others may take the view that hitting it harder is the way to at least partially overcome this. I think Justine is likely to take the latter road.
Very few are successful with that approach. In recent history, Agassi and Serena Williams are the only players I can think of to have been successful with a hit harder and flatter approach to RG.
Justine is a special case, given that of the current crop of WTA players only a handful are good on clay, so she should be able to succeed with whatever gamestyle she chooses.