Some newbie questions on ECS

Hi, I'm trying to use ECS to boost performance on computational intensive algorithm, and here I've some newbie questions:

Assume I've a big map with a lot of cities which are randomly connected with roads ( a big graph ).
So each city is an entity, each road is an entity too.

Code (CSharp):

1. struct CpCity : IComponentData
2. {
3.      float population;
4. }
5.  
6. struct CpRoad : IComponentData
7. {
8.      Entity cityA, cityB;
9.      float cost;
10. }
11.  
12. void ChangeCost(Entity cityA, Entity cityB, float newCost) { ... }
13.  

Now I need to write the method to modify the cost of road given two cities.
Assume there'll be 100+ calls of the method per frame, and there are around 100K roads in total. Is it true that the only way is to use a NativeHashMap< Entity_City_Pair, Entity_Road > in a ComponentSystem?