You'll feel major flex on this type of bike, especially at speed. That is cumulative flex in the frame tubes. If you aren't a bantamweight, you can probably cause the head tube to waggle by flexing the handlebars. Part of the intrinsic charm of a vitus-like bike.

I think that if you regularly inspect the lug joints you should see a joint failure before it hurts, but I can't guarantee that. I'd ride gently down hills. They are unusual, but frame joint failures can be spectacular.