It's been this way since Kingmaker. Cast Haste and all your other /round buffs then speed through the dialog as fast as you can because those buffs are wearing off while you're talking.
E - so many people saying I'm wrong. I wish they were right. It's trivial to test. I fired up a save right before the succubus in the Gray Garrison and cast Divine Power (duration of 60s) and initiated the event with the crusaders and the succubus when Divine Power was around 55s left.
I casually went through the dialog, which took around 90s. When combat initiated, Divine Power had 38s left. So - not bad, lost 17s during the 90s of reading the text and the cutscene.
Then I tested again and sped through the text and cutscene by pressing space and enter when the camera would shift. When combat started I had 51s left on Divine Power, a difference of 2 rounds.
You want to casually take your time, then go ahead. But those buffs ar...