Are recommendations computed in real-time though? Have you considered e.g. batch recomputation overnight with a 'full' algorithm, and just applying a linearized model to any newly rated content?
I feel like the quality of the Netflix recommendations is not stellar, and if that's because you're constraining yourself to what can be calculated in real-time, I'd willingly trade-off having "perfect" real-time recommendations in favor of better recommendations tomorrow (with the full model). Even if you do try to update recommendations in real time, aren't they easily cacheable if you can't keep up? (Well, as easily cacheable as any dataset on 25 million subscribers can be...)
Some stuff is in real time, some is pre-calculated. There is an enormous amount of research and testing going on in this space all the time, its complex and it's evolving fast.
I feel like the quality of the Netflix recommendations is not stellar, and if that's because you're constraining yourself to what can be calculated in real-time, I'd willingly trade-off having "perfect" real-time recommendations in favor of better recommendations tomorrow (with the full model). Even if you do try to update recommendations in real time, aren't they easily cacheable if you can't keep up? (Well, as easily cacheable as any dataset on 25 million subscribers can be...)