Here is 1 for you.
IF you like matematic lets hear it. (note that elemental invasion lasts less then a hour so this is just theorycrafting).
In full 264 gear it takes 20 min to solo portal (for me as hunter) and each district has 4.
20 min /per portal X 4 portals = 80 min /zone
There are 6 zones in each city.
6 zones X 80 min/zone = 480 min ---) 8 hours
Now lets add that for each citizen in zone you look for you lose 10 min. (20 citizens)
10 min/zone x 6 zones = 60 min ---) 1 hour
In total i would need 9 hours to solo it.
I come to conclusion why blizzard made portal on timers-- if there wont be any timer lower lvl characters quests couldt be handled too any major city on low populated servers.