I encourage you to go ahead and make the call. The first call is just a warning and doesn't have to be seconded. No harm.
I've played with a lot of slow players, warned a bunch, and have never seen one who took even close to thirty seconds after that.
Usually, this means they stop taking a stroll down every fairway on every throw to look at the basket. Which has nothing to do with distractions.
But, if - and this is purely theoretical because I've never seen a warning not work - they did continue to play slow, remember you do not have to convince the thrower the area was clear and free. If you and another player on the card agree they should have thrown sooner, the call stands. The next recourse for the player is to appeal to the TD.
The rules are set up so there is no cause for arguing among players on the card if you follow the procedures in the rules.
Over the course of my time playing PDGA tournaments (I don't play a ton each year, but I have played several each year for the last 14 years), I have voiced a warning 3 times. (Including a foot fault, back when the rule required a warning before a penalty on the next violation, and slow play once.) One of those times things worked out the way you describe, the player thanked me and amended his actions. We were all the happier for it. The other two times the player became disgruntled and made life miserable for everyone else on the card for the rest of the day and both other card-mates were ticked off at me for the misery that ensued.
In the last few years I have had a couple of conversations with someone on the card about the behavior or play or another person on the card wherein I tried to convince them that we should call a violation or give a warning the next time the player did that thing. Both times the other player discouraged me from doing it and told me he would not second it bc he didn't want to get involved. I have learned from this that if I start talking about rules violations, I am seen as the guy who doesn't want to have any fun and who takes all this stuff way too seriously.
In a tournament last year a player on my card reported an incorrect score for a hole. All the rest of us looked at each other and it was clear to me that I was the only one who was going to speak up. I simply asked, "Are you sure?" What followed was fully ten minutes of the guy walking around, gesticulating wildly, and yelling in general and at me in particular. By that time the card behind us had caught up and he pled his case to those 4 guys for a few more minutes. Finally the three of us agreed on his proper score and we recorded it. He glared at me for the rest of the round and no one spoke for those last several holes other than to say a score number. I should note that the next morning before the final round he came up to me and apologized, which was good, since we were playing together again.
I'm not saying that this is how it should be. I wish we could all just separate our feelings and self-identity from our disc golf play and call all the violations without emotional reactions and relational cost.
It's true that "the rules are set up so there is no cause for arguing among players," but I've rarely seen a call of any kind made during a round that didn't create lingering bad feelings for the rest of the round and sometimes into a future tournament. Sorry for the essay. I will continue to do my best to call the rules violations that I think will be enforceable and I'm definitely willing to be "that guy" on things like improper reporting of a score or other obvious errors.