//High level description

Create two list

InterestionsList (this only hold points and lines)

CreatedRoadList (this list hold the mesh or we can say this list hold the lines that already been created from InterestionsList)

loop through InterestionsList and every item in InterestionList create a intersection

get all the lines that are connected to that intersection

check has the lines been create if not then create the road and connect the road to the intersection

else just connect the road to the intersection

List<Interestion> InterestionsList;  
List<CreatedRoad> CreatedRoadList;

void CreateInterestion(List<Interestion> inters , int connectedroad)
{
   //build mesh 
}

void buildroad(float startX , float startY , float EndX , float EndY)
{
   // build mesh base on the input value
}

void ConnectInterstion(Mesh m , Lines line)
{
   Road road = new Road();
   //connect intersection and the road base on the road dir and position 
    CreatedRoadList.Add(road);
}

void Update()
{
  for(int i= 0 ; i< i<InterestionsList.count(); i++)
  {
    InterestionsList[i].getPosition();   
    Lines[] lines = InterestionsList[i].getConnectLine();

    CreateInterestion(InterestionsList[i].getPosition() , lines.count)
    for(int i= 0 ; i< i<lines.count(); i++)
    {
       for(int j =0 ; j<CreatedRoadList.count(); j++ )
       {
          if(lines.ID == CreatedRoadList[i].ID)
          {
             ConnectInterstion(m, Lines[i]);
             break;
          }
          else
          {
              Mesh m = buildroad(Lines[i].Start.x, Lines[i].Start.y ,Lines[i].End.x,Lines[i].End.y);
              ConnectInterstion(m, Lines[i]);
          }
       }
    }
    
  }
}

Some questions about it being "slow".

First, if possible, please give link to the actual source code.

How many items do you have in each of your lists?

How many times are you going through your lists?

For your description, you've got "Loop through, and for each, and if it hasn't been created, take an action" Beware of that kind of statement if you're looking for performance.

That looks like you've got a triple nested loop over the collections. If you also have a large number of items in that loop, it will be very slow. In computer science, a triple-nested loop like that would be called an O(n3) operation; the number of actions is the cube of the number of items. Consider if there were 10 items, that's a thousand actions. If there were 100 items, that's a million actions. If there were 1000 items, that's a billion actions.

Another concern is that making new objects is potentially a slow action, especially if your collection of items must grow many times.

These could be reviewed if you post a link to your actual code.