Whoa. Note the majority of the Grind present is of the "sometimes experience it variety". I voted that, since I'll generally run into the grind in one combat out of every 3 or 4 sessions. I can't speak for others, but I think you might be making some rather unreliable assumptions about your results.
Such as?
Now, even aside from that, 25% of players regularly experiencing grind is an issue. What I would be curious about, for a more informed poll, was knowing how large some of those groups are, along with whether they have experienced grind in other games or other editions.
Yes, more details would be good. But even so, the game system should be able to handle 6 PCs or even 7 PCs without grinding. Shouldn't it?
I know that for myself, when grind does happen it tends to be partly due to group size and player inactivity.
In regard to group size, why is just adding one or two players detrimental to the game system to the extent that 2 in 9 players often experience Grind (if one were to make an assumption that group size is a major contributing factor)?
When it arises as a result of the rules, it tends to be the result of specific mechanical issues - likely a combat full of insubstantial creatures, for example - more than general hitpoints and book-keeping.
No doubt that insubstantial adds to the issue. By definition, that effectively doubles the hit points of the creature if it can be insubstantial most of the time.
But, insubstantial is a game mechanics feature. So my statement that you responded "Whoa" to: "it seems likely that Grind is at least partially mechanically inherent to the rule system itself" is reinforced by game mechanics like Stun, Insubstantial, Blind, Weakened, Regeneration, etc. And since percentage-wise, more PCs and NPCs now have these types of abilities than in earlier editions, by definition these types of game mechanics are adding to the duration of encounters. Both by adding rounds to the length of an encounter, and by adding bookkeeping which players have to keep track of.
Stun is especially Grind conducive. I have seen a player miss his save for Stun 4 or more times in a row, effectively taking that PC and player almost completely out of an encounter.
But consider. What if 4E had a simpler duration system? All Ally beneficial non-zone effects last to the end of the encounter. All Enemy detrimental non-zone effects last until the enemy makes a save at the end of his turn. How much real time would this save both in doing bookkeeping, and in things like forgetting that an effect expired? In a one hour encounter, wouldn't it be possible for a rules change like this to save 5 or more minutes?
As players, some of us get used to the system as we play it and do not really take out the time to think about a better system.
But I think that Grind is not caused by just one thing, but a combination of many things that add up. Effects like Stun or Insubstantial can add to Grind, doing excessive bookkeeping can add to Grind, high number of monster hit points can add to Grind, slow players can add to Grind, larger groups can add to Grind, more challenging encounters can add to Grind, traps can add to Grind, etc.
I have seen traps that require Teleport or one of a few skills for the PC to get out of the trap. If the PC doesn't have that ability, it's possible that other PCs might have difficulty (or even lack of time) helping out that PC. The player might experience Grind as rounds go by and s/he cannot make a skill roll and is not contributing to the encounter. I've seen this happen in Skill Challenges as well.
Most players want to contribute. Game elements and mechanics that minimize their contributions can be perceived as grindy.
When 3E came out, it introduced a circular initiative system. This was a vast improvement over earlier systems that required an initiative roll every round. Bookkeeping adds up and is just one factor in the Grind equation, even if it is not as obvious to some people.