[psandersen]

The feature engineering is probably where most of the performance comes from so there is likely a lot of code that turns the raw data into features.

Features likely include more than just the single users history, so they need to be updated often enough for the model to do fast predictions. E.g. you want your model to capture if many people are booking from the same area at once because there were results from a sports game etc, but you dont want to run an expensive query for every user of the page.

Definitely not the first thing to worry about in a startup, but better performance at Booking.com's scale is serious $$$.