Characters walking into the city seem to have no particular problems, but those using a portal such as the one in Undercity or in the Pandaria Shrine, appear to be having issues. On attempting to confirm this, the portal from Undercity was used.
I monitored my frame rate across Orgrimmar, onto the Zeppelin, over to Undercity from Brill and out of the portal room in Silvermoon to the scene you see in front of you. You can see the results of this to the right-hand side, the 31 FPS is when I first arrived in Silvermoon, dropping to 10 when I stepped outside the portal room itself, then to four, and then stabilizing at 2FPS, where it remained for 15 minutes until I was unceremoniously disconnected. A second attempt with no addons revealed the same results, though marginally higher frame rates.
Silvermoon is very graphically intensive, and I'd turned my graphics down from their usual Ultra to Good in anticipation of issues like the ones experienced. On logging back in after my disconnect, I had a framerate of zero even with lower graphics settings, so dropped the quality to Low, which gave me a framerate of 6.