Best time to go through Seattle on I-5 is in the middle of the night. It's getting to be a******shoot any other time - sometimes it's okay, sometimes it's very bad. Often there is no rhyme or reason, so I avoid Seattle now. Same on the east side (I-405). If you have 2 or more in your vehicle then use the "diamond" lane.
Suggestion: check the Mariner schedule if heading up or down I-5. If there's a game at Safeco stay away from I-5. Especially this weekend: the Red Sox are in town.
I used to give cogent advice about alternate routes, but not anymore. Too many people, too much traffic, the same amount of roads, and plenty of construction.
Good luck.