Nice. It's a rare way to play these days and I kind of miss it.I track in-world game time very carefully, and not just during encounters. How long it takes PCs to get from point A to point B could easily impact the adventure and what they encounter when they arrive.
I track time during dungeon crawls so I know when longer spells (See Invisibility and even Mage Armor) run out.
I use the old 10-minute turn-based counter (a d6) and a d12 for an hour counter, whenever either are appropriate, otherwise I just keep a mental estimate of elapsed time.
But, our groups also track things strictly like encumbrance, rations, weather conditions, etc. to make sure the PC can do the things they want. If weather is rough (like blizzards in Frostmaiden) it can easily affect an encounter (ranged attacks, encounter distance, etc.). This is more important at lower levels, of course, but can still impact game play at higher levels as well.
I run a lot at conventions and I usually run an ongoing sandbox game (one year I did 6 sessions of Isle of Dread in 5E for example). It turns out that in a 4 hour slot that includes people choosing characters, a break or two and the sorts of tomfoolery you find at a convention table, there is always a good bit of exploration, a few skill challenges, perhaps a trivial fight and very often a single big, dynamic battle (with dinosaurs! in this case). Which, for me, is a perfectly fine mix. But even at mid levels 5E characters can really bring the hurt if they feel free to nova.So, then the "one fights" are typically Hard or Deadly I would think? It overcomes the more trivial easy and some moderate encounters, which otherwise exist (outside of story reasons) simply for the sake of resource attrition. I've considered that myself.