For anyone wondering why: I was working on a hobby project a few years ago that had to do a lot of inserts in a SQLite DB. Performance was ok, but it wasn't great either. Turning on WAL greatly sped up performance. In both cases just a single thread was writing to the database.
With the WAL turned off (the default), when you start a transaction SQLite will duplicate the data into a new file, change the original file, and delete the backup file when the transaction is committed. With WAL turned on it will write each change to a journal file, and merge this into the main file periodically (by default when the change file grows to 1000 entries) - so only a single write is needed.
With the WAL turned off (the default), when you start a transaction SQLite will duplicate the data into a new file, change the original file, and delete the backup file when the transaction is committed. With WAL turned on it will write each change to a journal file, and merge this into the main file periodically (by default when the change file grows to 1000 entries) - so only a single write is needed.