The best time for pruning Forsythia is as soon as the flower dies off and the leaves begin to grow. This timing is essential if you want a flowing natural look.
Longwood Gardens in Pennsylvania prunes by cutting the whole plant right down to the ground every 4 or 5 years right after the bloom. This practice keeps the plant looking fresh without old branches building up.
This chopped look bothers me. It is the result of pruning whenever you think about it. Please grow a beautiful garden by pruning correctly.