SQLite Forum

How to append one table of one database to another table of another database ?
I have a memory database as a buffer, and I'd like to periodically copy one of its table to the file database. 
The table schema of both file and mem db are identical.  
I tried to use INSERT INTO fileDB.table (columns) SELECT (columns) from memDB.table. Although it worked well, but the performance was slow. 
Is there any other method for example copy data from pager layer that may improve the performance ? I looked the backup API, it seems only copy the full database to another file and make them the same. How to append one to another ?