I ran into similar issues recently. A lot depends on your SQLite configuration - could you share yours?
You should keep in mind that only one connection can write to the database, while multiple concurrent reads are possible, but only under certain settings.
For example, if you rely on a **:memory:**db, you may need to set read_uncommitted: 1 to allow reads not to be blocked by writes (though be aware that reads may then see uncommitted changes) otherwise all concurrent read operations will be blocked by every single write.
database: “file:db=memory&cache=shared”,
pool_size: 50,
//read_uncommitted allows reads to proceed without waiting for write locks,
// improving read concurrency at the cost of
// potentially reading uncommitted data
custom_pragmas: [read_uncommitted: 1]
If you’re using a file-based database, you should set the journal_mode to WAL (which is usually the default).
database: “priv/data/sqlite.db”,
journal_mode: :wal,
pool_size: 50,
Another important aspect is caching. By caching the output of a SQLite query for just 1 second, I was able to increase throughput from about 1,000 reads/sec to around 2,000,000 reads/sec 
