Slow performance with googlemaps js api v3

I started using a marker cluster vor google maps some time ago http://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclustererplus/docs/reference.html and used my own markers by extending googles OverlayView
this.extend(CustomMarker, google.maps.OverlayView);
now it happened that this was incredibly slow with around 3000 markers. I have seen other sites rendering way more than 3000 markers and they were fast.

To keep it short. Just be aware of the fact that chrome's developer tools can have huge performance impacts. so closing dev tools will give you a hint whether you are dealing with a performance issue or just having the tools open