FYI `schemaCompletionSource` exists https://github.com/codemirror/lang-sql#user-content-schemaco...
But if you want an example of async completion, I do it here
https://github.com/AlexErrant/Pentive/blob/fe86a8a185e4be470...
which is called from here
https://github.com/AlexErrant/Pentive/blob/fe86a8a185e4be470...
Perf isn't really a problem for me since I'm calling SQLite in the browser; it's maybe 10-20ms.