The BBEG wants to delay the party in the final battle… (narrative delay?)

I have thought of a few methods they might accomplish this. In this question, I am wondering about a potentially more controversial option. I am considering having them attempt to monologue or engage the PC’s in dialogue (e.g. offer for them to join his side, explain his plan, answer questions, etc) with a timer running, then increase the battle rounds based on the amount of time they were able to enthrall the PC’s with their speech.

I can see some problems with this:

  • I haven’t used this "on the clock" method before and the players may not recognize what’s happening (kind of the point?); I’ve loosely enforced limiting speech to your turn to maintain a suspension of disbelief
  • This could cause balance issues with planning the time depending on if they catch on quickly or not as all (see below)
  • It would require significant setup, coordinating clues that may let them read into what is happening while also maintaining a monologue
  • Our game is online, so it may be more difficult to implement this strategy

Some details about the campaign/fight (major spoilers for Paizo’s Ruins of Azlant AP)

The party will already be in initiative when they arrive in his area (they have to fight or bypass a couple ‘mooks’ before reaching him) but in our circle of gamers there is a precedent for NPC’s to interrupt combat rounds with speech. In this campaign, that even includes the players taking up another potential enemy’s offer of working together instead of fighting. In the past, however, it hasn’t been relevant that the clock continues moving.

My goal is to cost them a handful of rounds if they are willing to listen to the creature, who is manipulative by nature. I feel this is thematic but unlikely to make the difference in their success or failure in regards to the creature’s plan, but I think it would be an epic moment if it does (or comes down to the line).

Is this a bad idea? Has anyone had success doing something like this, and what was needed to pull it off?

Directly related to my question about doing it with spells.

Reminder: per "Good Subjective, Bad Subjective", to answer this question, one would either need direct experience using such a delay tactic or have experienced a GM doing so with them as a player.