Optimize QSql of memory database in multiprocessor-thread

Here's my problem, I have one sqlite memory database by using QSql. I have several threads each handling one different table of this common database. And I use Win API to make sure these threads working on different processor, like this:

SetThreadAffinityMask (hThread, processorMask);

When there's only thread handling one table, it takes 10 seconds and uses 25% of total CPU. But when there're 4 threads handling 4 different tables, it takes nearly 40 seconds and uses only 35% of total CPU. I think the reason is there's some kind of thread-safe sync in one database. But due to different thread reading or writing different table, thread-safe slows my program. How can I optimize it.

Update: The most possible reason is some kinds of lock inside of Qt or/and Sqlite 3 slows my program, so is it possible to shutdown or bypass these locks by pre-setting.

Update2: Here's an example. (Maybe a little long, sorry)

class MultiProcessorThread
{
public:
    virtual void run();
    bool start()
    {
        m_hThread = CreateThread (NULL, 0, MultiProcessorThread::ThreadFunc, this, CREATE_SUSPENDED, NULL);

        if (m_hThread != INVALID_HANDLE_VALUE)
        {
            RunningThreadCount++;
            m_ProcessorMask = 1 << ( (RunningThreadCount - 1) % ProcessorCount);
            SetThreadAffinityMask (m_hThread, m_ProcessorMask); // Make thread working on different processor
            ResumeThread (m_hThread);
            return true;
        }
        else
            return false;
    }
protected:
    static DWORD WINAPI ThreadFunc (LPVOID in);
    HANDLE m_hThread;
    DWORD_PTR m_ProcessorMask;
    static DWORD_PTR ProcessorCount;
    static DWORD_PTR RunningThreadCount;
    static DWORD_PTR GetNumCPUs();
};

DWORD_PTR MultiProcessorThread::ProcessorCount = GetNumCPUs();
DWORD_PTR MultiProcessorThread::RunningThreadCount = 0;
DWORD_PTR MultiProcessorThread::GetNumCPUs() // Get how many processors on this PC
{
    SYSTEM_INFO m_si = {0};
    GetSystemInfo (&m_si);
    return (DWORD_PTR) m_si.dwNumberOfProcessors;
}
DWORD WINAPI MultiProcessorThread::ThreadFunc (LPVOID in)
{
    static_cast<MultiProcessorThread*> (in)->run();
    return 0;
}

class Run : public MultiProcessorThread
{
public:
    void run()
    {
        int i = 0;
        QString add = "insert into %1 values(1)";
        add = add.arg (table);
        QString sel = "select a from %1 ";
        sel = sel.arg (table);
        QString del = "delete from %1 where a=1";
        del = del.arg (table);

        while (++i) // read and write database
        {
            query.exec (add);
            query.exec (sel);
            query.exec (del);
        }
    }
    QSqlQuery query;
    QString table;
};  

int main (int argc, char *argv[])
{
    QCoreApplication a (argc, argv);
    QSqlDatabase db = QSqlDatabase::addDatabase ("QSQLITE", "test"); 
    db.setDatabaseName (":memory:"); // All threads working on the same memory database.
    db.open();
    QSqlQuery q (db), q1 (db), q2 (db);
    q.exec ("create table A (a)");
    q1.exec ("create table B (a)");
    q2.exec ("create table C (a)"); // All threads working on different table.
    Run b[3];
    b[0].query = QSqlQuery (q);
    b[0].table = "A";
    b[1].query = QSqlQuery (q1);
    b[1].table = "B";
    b[2].query = QSqlQuery (q2);
    b[2].table = "C";
    b[0].start();
    b[1].start();
    b[2].start();
    return a.exec();
}