Game UI becomes less responsive over time

After keeping the game open long enough, it starts using more CPU and becomes less responsive. Refreshing returns performance back to normal. This appears to be linked to the growing chat log, since it's more prominent when chat is active (sometimes becoming practically unusable after only being open for 1-2 hours). Periodically clearing older chat messages (perhaps keeping only ~2 screenfuls of messages in combination with chat history) might help this.