Well, I’ve just had 2 completely different experiences at SEA over the last couple of weeks.

The week before Xmas, arrived in the usual throng, was directed to ESTA line and waited…and waited…90 minutes from exiting plane to exiting immigration and customs. Only half the booths manned.

Second time on New Year’s Day, arriving from Canada at roughly the same time as our U.K. flight did (no pre-clearance at Kelowna so it was a like for like experience). Immigration booths again only 50% manned, immigration hall rammed. ESTA queue looked about 90 min - 2 hours.

Was advised to download the mobile passport control app which has now been extended to visa waiver passport holders. Filled in the online form, went to the MPC queue and was through in about 10 minutes.