Not too busy. Small
Most people don't know,,, that by new rules SFO's 2 main runways are too close together, not 1000ft apart. Thus, with fog or clouds or pilots without 'instrument rating', some flights are diverted. Also big Asiana crash and huge Sir Canada near crash of 4-5 airplanes when the pilot Almost landed on the Full taxi way instead of the Wide Runway!! Oakland airport is considered Much Safer, if you care.
A few Saturdays ago our Alaska airlines plane that was supposed to land at San Francisco was diverted to San Jose. This was mid afternoon on a Saturday, and it did not seem that busy. It was quite easy to get through the airport and take an Uber to our final destination downtown San Francisco
It has been busy every time I have a lay over from Boise too Burbank and that's usually at night. There is a good selection of food, snacks,drinks, charging station and a place for your Pup if you travel with one. Its not a small airport in my opinion Of course not big like Lax..
Pretty busy.
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