The resting heart rate of an “athletic”, middle-aged human male (on earth, anyway) can dip down to 47 BPM or so. For well-trained, battle-ready soldiers 10 heart beats could work out to as much as 12-13 seconds, which is a lot longer than it sounds if people talk fast (and they don’t need to stop & think about what they’re going to say). That said, you aren’t wrong…
The first time Sanderson does the “10 heat beats [a bunch of stuff happens]” thing, back when the chasmfiend attacks, I thought the “time stretching” effect was a great way to pull you into the scene (IIRC, people in suddenly dangerous situations have reported feeling like time slowed down), even if the timing was a bit long. I remember at least one other occurrence where Sanderson clearly didn’t keep track of time correctly. Unless, of course, Rosharans have lower heart rates than humans do IRL. Or maybe he doesn’t care and is just doing it for dramatic effect… having a countdown started like that does tend to add a sense of urgency to the situation.