Forsythia plants are one of those plants that benefit from hard pruning every few years. It keeps the stems freash and help to produce lots of new wood to carry more flowers in the second year of growth.
Your Pruning program for pruning Forsythia should be to after its flowered in the spring remove the old stems on a 4 year cycle i.e removing the stems as they become 4 years old and then every 6 years prune the whole plant hard back to to regenerate it.
It you have an overgown Forsythia its possible to prune it back almost to the ground i.e 200mm and it will re-shoot from dormant buds in the stem. The re-shooting can take upto six months to happen depending on the time of the year the pruning is carried out