No it is not slippery. The floor is sloped from the glass to the back wall ¼” per foot so it feels relatively flat and still drains great. It also helps that the marble is honed. I have installed polished marble with the same application in a steam room and it was not slippery either.

Very helpful feedback, thanks. I've heard from too many small tile "traditionalists" that larger marble tile (honed, or polished) is too slippery in a walk-in shower. This helps me confirm that's simply not true!

Do you by any chance know how wide the grout lines were? I'm trying to figure out if we can get away with 1/16" to minimize mold/mildew risk.

In this picture the grout joints are 1/16”. One of the reasons why I love doing linear drains is because it minimizes the grout joints. This is because you can use large format tiles on a shower floor. If you are nervous about having a porous stone on a shower floor, then seal it with Porous Plus made by Miracle, or there are porcelain tiles that have the same look as Carrara Marble and then you can use epoxy grout. The porcelain tile and epoxy grout combo is impervious to water and staining. If you would like to speak with a couple of my clients that have large Carrara Marble tiles on their shower floor with 1/16” grout joints, honed or polished, please let me know and I will be happy to provide you with their contact info