I’ve found that the city I’ve built at Telith’s Island is infinitely laggier than cities elsewhere, and it’s reasonable to assume that environment has something to do with it.
It’s a particularly heavily wooded area, then there’s the lake, thunderstorms, sandstorms (which can blow by at the same time as thunderstorms) and all manner of other things.
One thing I’ve noticed which cant be helping is the perpetual motion of plants and trees. I have flagpoles outside my gates, and even when the flags aren’t moving, as there’s no wind, the plants and trees continue very exaggerated movements, like there’s a gale blowing, with the trees particularly moving in a way which could induce sea-sickness! 
There’s a lot of trees permanently visible in that area. Maybe reduce the motion a touch or have them react to the wind the same way flags do. Might reduce a bit of load on the system.