DuckDB is heavily focused on analytical workflows, so it stores data in a compressed columnar format. So "rewrite one row" is more efficient in SQLite, but "sum all the values in this column" is vastly more efficient in DuckDB.
That's the core of the difference, to me. But the projects also have a pretty different ethos. SQLite has always prioritized simplicity, often at the expense of features, whereas DuckDB's approach to features seems to be "the more the merrier." E.g. the star feature of this new release, client/server mode. Plenty of people would love to see something similar in SQLite, but it doesn't seem likely.
That's the core of the difference, to me. But the projects also have a pretty different ethos. SQLite has always prioritized simplicity, often at the expense of features, whereas DuckDB's approach to features seems to be "the more the merrier." E.g. the star feature of this new release, client/server mode. Plenty of people would love to see something similar in SQLite, but it doesn't seem likely.