In that case my recommendation to take US-97 is even stronger. From Blaine it is pretty easy to take I-90 east out of Seattle and pick up US-97 at Yakima. Stay on 97 all the way to Weed, CA. This avoids the metro areas of Tacoma, Olympia, and Portland which all have famously bad traffic.
I-5 in Oregon from Portland to Eugene can be a parking lot any day of the week. The speed limit is 55-65 and you'll be lucky to even go that fast.
Over on US-97 the speed limits are also 55-65, but with a tiny fraction of the traffic. I think 97 might actually have more miles signed at 65mph nowadays than I-5. The only cities you have to go through are Madras, Redmond, Bend, and Klamath Falls, and you can get through all of them pretty quickly.
The only time you can rely on I-5 being faster is in the middle of the night.