Grid the landscape, and shortcircuit checks on the collision array if there are no nearby checked in collisions to be collided with. This is a big win for fire's ammoshove in particular. Also add a +2 that seemed missing in the check, and update fire accordingly.
#include <QLabel>
#include <QRect>
#include <QList>
#include <QMouseEvent>
class PixLabel : public QLabel
{
Q_OBJECT
public:
PixLabel();
QList<QRect> rects;
public slots:
void AddRect();
private:
void paintEvent(QPaintEvent * event);
void mousePressEvent(QMouseEvent * e);
};