It took Gegard about 5-10 seconds to fully sink that triangle in, which Kang could have easily pulled out of because it wasn't tight at first. Then, we all know what happened there.
I'm not downplaying anything, I'm just saying that a BJJ blackbelt shouldn't be a freakin' dipshit and let someone who's known as a striker slap on a slow triangle.
First of all, I think you're both exaggerating how slow the triangle was. It only took Gegard about 3 seconds to lock the triangle around Kang's neck (in order to close off his escape and control his posture), and it only took him around 8-9 to submit him. If you don't believe me, here's the .gif with a timer:
Denis had only 3 seconds to avoid the triangle, because once Mousasi had his arm, the triangle was virtually already locked-in. Sure, he should have seen it coming, but with Mousasi not being notorious for subbing people off his back, I wouldn't be surprised if Denis was completely caught off-guard. Not that that's an excuse, seeing as he's a black-belt in BJJ, but the triangle wasn't nearly as slow as Hominick/Edwards.
Mousasi's skillset is improving, this is really undeniable.