X devAPI batch insert extremely slow – Mysql

I use the C++ connector for MySQL and the X dev API code.

On my test server (my machine), doing a single insert in loop is pretty slow (about 1000 per second) on a basic table with a few columns. It has a unique index on a char(40) field which is possibly the cause of the slowness. But since the DB is configured as developer mode, I guess this should be expected.

I wanted to improve this by doing batch inserts. The problem is that it is even slower (about 20 per second). The execute() itself is quite fast, but the .values() are extremely slow. The code looks something like this:

try
{
    mysqlx::TableInsert MyInsert = m_DBRegisterConnection->GetSchema()->getTable("MyTable").insert("UniqueID", "This", "AndThat");
    for (int i = 0; i < ToBeInserted; i++)
    {
        MyInsert = MyInsert.values(m_MyQueue.getAt(i)->InsertValues[0],
            m_MyQueue.getAt(i)->InsertValues[1],
            m_MyQueue.getAt(i)->InsertValues[2]);
    }
    MyInsert.execute();
}
catch (std::exception& e)
{
}

Here is the table create:

CREATE TABLE `players` (
  `id` bigint NOT NULL AUTO_INCREMENT,
  `UniqueID` char(32) CHARACTER SET ascii COLLATE ascii_general_ci NOT NULL,
  `PlayerID` varchar(500) DEFAULT NULL,
  `Email` varchar(255) DEFAULT NULL,
  `Password` varchar(63) DEFAULT NULL,
  `CodeEmailValidation` int DEFAULT NULL,
  `CodeDateGenerated` datetime DEFAULT NULL,
  `LastLogin` datetime NOT NULL,
  `Validated` tinyint DEFAULT '0',
  `DateCreated` datetime NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `UniqueID_UNIQUE` (`UniqueID`)
) ENGINE=InnoDB AUTO_INCREMENT=21124342 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

Any clue why this is much slower? Is there a better way to do a batch insert?