You are right in the fact that the X denotes the start of the speed point, but as I said the shaded square differentiates the point from other points along the track. If you look at the legend on the STAR plates themselves they are quite specific. The SLP is the shaded square.........
X is simply a distance along track
Shaded Square is a SLP.
The fact that the speed is written with an arrow is purely because they cant fit a font small enough inside the SLP box to say what the speed is. SLP'scan be 250kts, 220kts 200kts etc.........
Cheers