WizardOfFrobozz
Accardi-by-the-Sea
Totally agree wrt dropping in-game clues as to what's happening ("You're quick, but he's... extraordinarily fast. It's hard enough to keep him in view, let alone hit him.") But I've been trying to keep the OOC discussion out of it as much as possible, at least in the moment. It really snuffs out the excitement for most of the table to turn everything into a spreadsheet, so to speak.I don't usually tell them while the encounter is ongoing, but I might deconstruct it a bit for my players afterward in the interests of getting people to know and understand the system. But for other ones, I'll say what it is like legendary actions or legendary resistance - just so they have some feedback on what's going on. Otherwise, it can be frustrating to have no clues why something is happening.
If I had a player nagging at me about why a hasted enemy was getting an extra attack or dash, I'd tell them, "Yeah, they did get another attack. That's interesting. Now it's up to you to figure out how!" I also might describe how quickly the NPC seems to be moving, again, so there's some clue on why there's a difference.
One takeaway I have from your post, though, is that it might help if I explicitly defer it rather than just trying to sidestep it. I can definitely lean into that, e.g. "Let's debrief on the mechanics at the next break, OK?"