。
Your brilliant lecture was pearls before swine.
Literal
Your [possessive-の] hard-won great-lecture [topic-は] gold-coins to cats was [confirmation-ね].
猫に小判 'gold coins to a cat' is the Japanese equivalent of 'pearls before swine' — giving something valuable to someone who can't appreciate it. せっかくの 'hard-won, painstakingly obtained' adds sympathy for the wasted effort. 名講義 'brilliant lecture.' A sympathetic, slightly teasing observation.