90 to 5 can be a major PITA, depending on the time of day. You have to go under the convention center and multiple lanes are merging into fewer lanes. Not exactly a smoothly flowing area at the best of times. It can be slow until north Seattle. You can also slow down around Everett.
I like the person's idea of crossing into Canada earlier than going up 5, and heading west into Chilliwack via the Trans Canadian Highway. Might take longer (heck if I know; just guessing) but it sure would be more scenic.
Also, depending on when you come over, there might be some highway closures for road work (getting to that time of year!). And they're still doing some work on the new 520 floating bridge across Lake Washington (it's closed this weekend for grand opening ceremonies, exhibits and what-not, plus folks can walk across it), so from time to time it's shut down, which puts more traffic on 90 or 405 heading north.
Just too many people and not enough roads. And it won't really get much better as they're not planning on new freeways but want mass transit (light rail mainly) to take up the slack. I'll be dead by the time they get it all done!