game slows down when I click on maximize window

[qwe522y]

hello;
When I click on maximize window my app slows down becose of large workspace(1080x1900);
help me to solve this problem;
my gameloop is correct or no? thanks;
ubuntu 12.04 KDE4 Qt5.1

###########main.qml##############
import QtQuick 2.0
Rectangle {
    width: 360
    height: 360
    focus: true
    Tank{id: myTank; objectName: "myTank"; x: 100; y: 100}
    Timer {
        running: true
        repeat: true
        interval: 16
        onTriggered: {
            myTank.move();
        }
    }
    Keys.onUpPressed: { myTank.moveUp = true}
    Keys.onDownPressed: { myTank.moveDown = true }
    Keys.onLeftPressed: { myTank.rotateLeft = true }
    Keys.onRightPressed: { myTank.rotateRight = true }
    Keys.onReleased: {
        switch(event.key) {
        case Qt.Key_Up:
            myTank.moveUp = false
            break
        case Qt.Key_Down:
            myTank.moveDown = false
            break
        case Qt.Key_Left:
            myTank.rotateLeft = false
            break
        case Qt.Key_Right:
            myTank.rotateRight = false
        }
    }
}
##############Tank.qml##############
import QtQuick 2.0
Item {
    width: 30
    height: 30
    property bool rotateLeft: false
    property bool moveUp: false
    property bool rotateRight: false
    property bool moveDown: false
    property double xpos: x
    property double ypos: y
    property double rspeed: 3
    property double speed: 3
    rotation: 120
    function move() {
        if(rotateLeft) rotation -= rspeed
        else if(rotateRight) rotation += rspeed
        var diffX = speed * Math.sin(rotation * 0.0174532925);
        var diffY = speed * Math.cos(rotation * 0.0174532925)
        if(moveUp) {
            xpos += diffX
            ypos -= diffY
        } else if(moveDown) {
            xpos -= diffX
            ypos += diffY
        }
        x = xpos
        y = ypos
    }
    Image {
        anchors.fill: parent
        source: "qrc:///Battle_City_Tank_Player1.png"
    }
}

###########main.qml##############
import QtQuick 2.0
Rectangle {
    width: 360
    height: 360
    focus: true
    Tank{id: myTank; objectName: "myTank"; x: 100; y: 100}
    Timer {
        running: true
        repeat: true
        interval: 16
        onTriggered: {
            myTank.move();
        }
    }
    Keys.onUpPressed: { myTank.moveUp = true}
    Keys.onDownPressed: { myTank.moveDown = true }
    Keys.onLeftPressed: { myTank.rotateLeft = true }
    Keys.onRightPressed: { myTank.rotateRight = true }
    Keys.onReleased: {
        switch(event.key) {
        case Qt.Key_Up:
            myTank.moveUp = false
            break
        case Qt.Key_Down:
            myTank.moveDown = false
            break
        case Qt.Key_Left:
            myTank.rotateLeft = false
            break
        case Qt.Key_Right:
            myTank.rotateRight = false
        }
    }
}
##############Tank.qml##############
import QtQuick 2.0
Item {
    width: 30
    height: 30
    property bool rotateLeft: false
    property bool moveUp: false
    property bool rotateRight: false
    property bool moveDown: false
    property double xpos: x
    property double ypos: y
    property double rspeed: 3
    property double speed: 3
    rotation: 120
    function move() {
        if(rotateLeft) rotation -= rspeed
        else if(rotateRight) rotation += rspeed
        var diffX = speed * Math.sin(rotation * 0.0174532925);
        var diffY = speed * Math.cos(rotation * 0.0174532925)
        if(moveUp) {
            xpos += diffX
            ypos -= diffY
        } else if(moveDown) {
            xpos -= diffX
            ypos += diffY
        }
        x = xpos
        y = ypos
    }
    Image {
        anchors.fill: parent
        source: "qrc:///Battle_City_Tank_Player1.png"
    }
}

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