ShortestWalk ghcomp index partition question

Hi, I'm seeking some help with this simple script. As the image shows, I want my segment indices to be partitioned by each route. This is no problem. However, when I script this partition with a simple list comprehension the response time jumps from around 1.3 seconds to 11.9 seconds. I include the ShortWalk component for comparison, which achieves both in under a second. I need this script to be inside a large code, is there any way I can achieve the correct partition without almost 10x the overhead? Is there a problem with my list comprehension? Images and file included, any guidance would be appreciated, thanks. 

edit: fixed an error