I normally prefer around 3 hours but you should be ok as long as your first flight is on time and don't dawdle.
In case you don't know you'll firstly clear US Immigration, then collect bags, clear US Customs, return bags to the airline and get to your departure gate - any delays are likely to be with US Immigration rather than US Customs where you normally just handover your declaration form.
Try to sit as near to the front of the plane on your flight from Dubai to improve your chances - as an aside I assume you have your ESTA for entry to the US.
I have assumed your flights are on a single ticket - if so and you miss the flight then the airline will get you on the next flight with available seats.
Clearing US Customs in Seattle is the worst that there is. You think that 2 1/2 hours is plenty of time but at Seattle it may not be anywhere near enough time. I have miss my connecting flights more times out of Seattle than I have anywhere in the US and I carry a US passport and most of the time with military orders. They do nothing besides make people miss flights and delay them as much as possible. And to write and complain is a joke as well because they always say the same thing. It is to make sure that we are safe. Now come on we have already cleared customs in a different country and then I get home to be harassed by customs. I am as WHITE as any other white man can be. So you can not call this profiling it is call harassment and it needs to be stopped in Seattle. To give you an idea of how bad it is for me I flew to the east coast and cleared customs checked my bags and everything that needed to be done. It was done properly and very well. I walked outside and got scared because I thought I had forgot to do something because I had left the building so soon. While outside I got to talking to a guy like myself that traveled a lot and I asked him what I could have forgot and he laughed and asked me if I had ever flown into the east coast before and this was my first time on the east coast. And he told me he had flown into the west coast one time and never again because of this same problem. Seattle being the worst of the worst I now avoid Seattle all together and the west coast if possible because of the wasted time that customs like to do to their own people and not people from other countries not that they would deserve this either. I have no use for Seattle customs and in my opinion the whole bunch of them need to be replaced because of the better than you attitude. Bad news for them they are not better than anyone if anything they are failing to do their jobs properly.
I fly thru seattle alot never have had any problems or slowdowns
must just be your attitude.or if traveling alone white and middle age comming from asia they always profile for pedofile and do more questioning than average.
2 1/2 hours ought to be fine. I'm assuming this is all on one ticket?
I've never had a problem with Seattle ommigration and customs, including before getting my US citizenship. In fact it's one place where (as a US citizen) I'll book connections as low as 75 minutes.
2.5 hours should be fine with an on time arrival. Hopefully there won't be another plane load of pax that just beat you there, but even then I'd say 2.5 hours should be okay.
Note that in future it may get more crowded, as Delta is building up a hub there and will be adding more TPAC and TATL flights from next spring.
Hongjing1 - believe you are complaining about immigration, not customs.
I never have experienced any sort of problem at SEA. Maybe you should consider applying for Global Entry?
Obviously no one can guarantee. It is, of course, Immigration, not Customs, that normally is the delay. And that can vary depending by how many other international flights land at the same time. As noted above, whether or not both tickets were purchased together is important. Another is what airlines are involved, though transiting between terminals at SeaTac is usually not a long trek. What airlines are involved?
If the flights are on one itinerary and, if there are two airlines involved, the airlines involved have an interline agreement on the transfer of bags (most, other than Southwest, do), and you miss your connection, the airline will try to put you on the next flight with available seats to Reno, but that may not be the very next flight.
The statement that clearing Immigration and Customs is "the worse there is" seems a bit hyperbolic. Doesn't mean it's always quick but I'm sure there are many, many worst airports throughout the world.