FSX/P3D. It's one of the most difficult and resource hogging region in the whole of Flight Simulation. Even Seattle without a third party KSEA is a tough cookie to fly over yet alone into and out of. Add in Orbx FTX PNW and it's not much wonder why even Orbx has avoided developing airfields in the city of Seattle itself. As for FlyTampa, the original developers of the FS9 version of KSEA, the region creates serious mesh problems for Portland and vise-versa. These issues were very evident for those who owned FlightZone 02 Portland and FlyTampa Seattle together. Similar issues exist for the FSX/P3D versions. Needless to say, FlyTampa bowed out of developing a newer version.
Taxi2Gate has picked up the reigns and I think I can speak for the entire flight simulation community when I say we are all very curious to see how Taxi2gate has managed to overcome the aforementioned issues. As we already know, FSDT Vancouver and FlyTampa Toronto respectively are not super easy on the CPU. As for the visuals of KSEA thus far, you will get no complaint from me. Indeed, all eyes are on Taxi2Gate!