Thanks a lot!
I now use Leaflet and the Leaflet extensions under Wordpress. This runs absolutely smoothly and I can now create really nice maps with it without robbing foreign resources. Thanks for the push in the right direction.
Only one small question I still have, but it does not fit to topic: I have a complete route with all waypoints. Can I split the route so that I can export only the routes between the waypoints?
Currently I help myself by estimating in the GPX file where the individual route ends, and then delete everything else.