Speed up ViewBox panning. Noticeable when a large number of items are present.

pyqtgraph/graphicsItems/ViewBox/ViewBox.py

@@ -1266,8 +1266,10 @@ class ViewBox(GraphicsWidget):
                     ## update shape of scale box
                     self.updateScaleBox(ev.buttonDownPos(), ev.pos())
             else:
-                tr = dif*mask
-                tr = self.mapToView(tr) - self.mapToView(Point(0,0))
+                tr = self.childGroup.transform()
+                tr = fn.invertQTransform(tr)
+                tr = tr.map(dif*mask) - tr.map(Point(0,0))
+
                 x = tr.x() if mask[0] == 1 else None
                 y = tr.y() if mask[1] == 1 else None