Autocomplete looks like a lookup. Type a prefix, return matching strings, sort by popularity. A LIKE 'tr%' query and an ORDER BY.

Two facts destroy that:

It runs on every keystroke. Not once per search — once per character. Typing “dinner” issues six requests. Across 10 million users that is roughly 24,000 queries per second for a feature nobody considers a feature.

The budget is about 100 milliseconds. Facebook’s typeahead team put the threshold there: slower and the suggestions visibly lag your typing, which feels worse than having none at all. That budget covers the network round trip, so the server has perhaps a few tens of milliseconds.