It's actually not so much the fog at SFO - it is that SFO's runway pairs have substandard separation, so any time vis under five miles or / and the cloud base drop to under 2,100 feet (roughly double of what VFR minimums - 3 miles / 1,000 ft - are!) they have to go to single runway ops. That slows things down a lot, even if it's just rain or drizzle.