Not sure exactly why updating an input's value resets the cursor / insert position to the end — maybe it's the autofocus attribute? — but it's mildly disconcerting.

This doesn't come up as long as the user only types lowercase, because the Preact diffing algorithm detects no change and thus doesn't update the DOM, so it's not a huge problem.

main.js (minified) is currently 2.3 MB (286 KB gzip'ed). Only 18 KB (7 KB gzip'ed) of that is functional code; the other 99% is dictionary data, which is represented as a trie, encoded as JSON.

There are many optimizations that could reduce the bundle size, like preprocessing identical branches in the trie to refer to the same object (this might reduce browser memory consumption, too?) or other alternatives to JSON.

Even with such speedups, the dictionary will always be a massive part, and on slow connections it would be better UX to load basic vocabulary first, synchronously, and then the less useful (≈ less used) remainder later, asynchronously. Merging two tries won't be trivial, but I doubt it'd be enough to interrupt UI responsiveness.