The issue with SQLite and concurrent writers isn't that it's slow, it's that it just can't do it. WAL mode lets you have as many readers as you want concurrent with a single writer, but it doesn't give you multiple concurrent writers. If you really need concurrent writes, use PostgreSQL or another RDBMS.

In my experience, SQLite is likely to be faster when you have lots of reading. Being in-process gives SQLite a natural advantage in read-heavy situations.