Every Sql query is executed as a prepared statement. This can result in a performance problem, because the network traffic is doubled.

Example:
QSqlQuery query("SELECT true;");

will send the following statements to the MySql server:
Prepare [12] SELECT true
Execute [12] SELECT true

This is a regression from Qt 4.4, since with Qt 4.4 only this statement is sent to the server:
Query SELECT true