I still don't understand why you can't use transactions. A SELECT statement will still return up-to-date data, even in the middle of a transaction. A disk database with transactions will easily support 10,000 simple inserts a second, so you could skip the in-memory database. Anyway, here is a sample of using ATTACH. I doubt it's much faster than doing it in Qt, but it is a bit more elegant.
Qt Code: 
  1. void setupDiskDb()
  2. {
  3.     QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE", "disk");
  4.     db.setDatabaseName("CatchyName");
  5.     db.open();
  6.  
  7.     QSqlQuery query(db);
  8.     query.exec("CREATE TABLE T1(F1, F2, F3)");
  9. }
  10.  
  11. void doSqLiteTest2()
  12. {
  13.     QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE", "mem");
  14.     db.setDatabaseName(":memory:");
  15.     db.open();
  16.  
  17.     QSqlQuery query(db);
  18.     query.exec("CREATE TABLE T1(F1, F2, F3)");
  19.  
  20.     query.prepare("INSERT INTO T1 (F1, F2, F3) "
  21.                    "VALUES (:1, :2, :3)");
  22.  
  23.     for (int i = 0; i < 50000; i++)
  24.     {
  25.         query.bindValue(":1", i + 0);
  26.         query.bindValue(":2", i + 1);
  27.         query.bindValue(":3", i + 2);
  28.         query.exec();
  29.     }
  30.  
  31.     qDebug() << query.exec("ATTACH DATABASE 'CatchyName' AS Catchy");
  32.     qDebug() << query.exec("INSERT INTO Catchy.T1(F1, F2, F3) SELECT F1, F2, F3 FROM T1");
  33.  }
  34.  
  35. int main(int argc, char * argv[])
  36. {
  37.     QApplication a(argc, argv);
  38.  
  39.     setupDiskDb();
  40.     doSqLiteTest2();
  41.  
  42.     return a.exec();
  43. }
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