To pre-slice or not to pre-slice fajitas?
Marinating only works on the surface of the meat as salt is the only thing that can get inside the slice of meat. So I suggest marinating the meat sliced and then cooking it. Extra bonus - cooking time is shorter!

I have some extra tips, take if you want. A good idea is to cook it hot and quickly like in a wok, but if you don't have a wok then that's okay (I don't have one myself). That way you sear the outside of the slices and what juice there is in the small piece of meat should stay in. If you do cook at high heat remember to use an oil with a high smoking point, like avocado, ghee, lard, or coconut oil.