Prop map perftweaks

These are O(n) to O(1) perf fixes in the way that PropMap is used. This bumps up some speed in markers and clustering markers. I wanted to tweak PropMap before I go ahead replace the clustering logic.
even though .values is cached in PropMap, we make sure to only get it once here. O(n) fetch, and then O(n) within an range in the for loop below.