Author of the sqlite_async package here.

There isn't anything obviously wrong with how the lib is used - the writes are properly batched in a single transaction, but the performance is much slower than expected.

From a quick look, garbage collection appears to be a big overhead - it could be related to how data is passed between isolates. In this case, it may actually be faster to do writes in smaller batches, since the current approach may use a lot of memory for the batch. I'll have to test further to see whether that helps, or whether there are other optimizations for this case.