I think it comes down to needing to find the right balance and adjusting throughout the Rp. All the Rps I gm start off with a basic minimal 2 posts between your last post for a character. Now, that usually goes well and keeps a decent speed without being completely overwhelming. 

Now if things start to get too fast, I'll make an adjustment to up the minimal to 3, 4, or 5. (Temporarily) Tag everyone and announce it in the ooc. When things slow down I will lower it back to 2.