Doing all background database operations on a single dedicated thread rather than throwing the work onto a general background task queue serviced by a thread pool avoids the framework performing expensive locking for you:
http://developer.android.com/reference/android/database/sqlite/SQLiteDatabase.html#setLockingEnabled(boolean)

Note that AsyncTask uses a thread pool (up to 128 threads in Android 2.3.7) to do background work.