Slow (assumed to mean super low) is a very low resolution model that is used in Shadow of the Colossus for distant places.
Description
Map tool from the debug menu. Red shows lyr0, green lyr1, the rest will be slows.
The map of Shadow of the Colossus is divided into quadrants, and these quadrants are divided into 36 seamless chips. Usually, the four chips closest to the player will load the highest quality models (lyr0), and the 32 chips in a circle around the lyr0 will be loaded with the second highest (lyr1). Beyond those, all models loaded will be very low detail in order to conserve memory; those are the slow models.
Textures
These models use small 64x64 (and in the case of earlier builds, 128x128) textures of the quadrants seen from above, and some versions of the game even show older iterations of quadrants not present in said version. Below are maps made out of the slow textures from each version:
