Yes, but only when SW is very crowded, typically during a major event (like right when the first pre-cata introduction quests were released) which happens very rarely. My lag and even framerate really is tied to the size of the nearby crowd...
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Dalaran, unlike SW/IF/Ogr has a very high particle density -- everything looks very nice.
Also it is relatively open, so you can see much further.
The design of the older cities, in general your vision is limited.
An equal number of toons in Dalaran is much worse than elsewhere.
But that's not saying you cannot have a lot more in SW than is typical in Dal.
With there re-designing of cities like SW, I think the same issues may arise, the new SW is a much larger city than Dalaran and if a lot of people use it as home base I could see lag issues arise in the expansion. I had a quick look around on the PTR and there are a of of new added bits in SW.
Out of curiosity...
How much of the city is self contained districts, like the current city.
When in the Trade District, you cannot see Old Town, Dwarf Quarter or the Mage Area.
So even if those were all very high polygon counts, they wouldn't strain your system at all.
How much of the city is vast open space, like Shatt/Dalaran?
So we are expected to get a portal if we want to go to, say Dalaran? That or walk/fly?
Those boats and flying take forever =(