It's sort of intractable, if we're talking about something like Google search. If we assume Google's algorithm is slow, personalizing for everyone one earth will make it 8 billion times slower. If it takes 5 minutes to run, personalizing it, for one run, will take 76,000 years.

I'm sure it's not that slow, and can be adapted to not redo a bunch of identical work for every person, but the point is that certain algorithms scale poorly.