I’m not saying that playing 6 characters is slower than playing with 4 in the same encounter. I’m saying that rebalancing the game for a default party of 6 would slow down all the encounters. To balance each fight against 50% more characters means either 50% more enemies, or make those enemies tougher. Either way should make the fights longer.
Except that's not how CR balance works in D&D at all. The weight of a given monster's abilities can be thought of as the base value of difficulty. The number of opponents can be thought of more as an exponent and less a multiplier. A given encounter will generally be "unfair" from a system standpoint versus the party. For example, enemies get multiattack first before players. This is because CR is based on a 1 versus "the party" dynamic.
Let's say we have 8 goblins. From a pure "add the CR" standpoint, that comes out to a CR of 2. You could look at this and think "great, a party level of 2nd level adventurers could knock this out of the park." Except this is a large encounter. That changes the math. The DMG says this should be multiplied by 2.5. So this becomes a CR 4.5. If I were designing this encounter I'd need to weigh the individual abilities of the creatures and make a judgement call on whether or not to treat it like a CR 4 or 5. These are plain gobbos so their only real threat is being engaged from a distance with shortbows. EXCEPT I know the terrain and it's going to take the players a couple turns to get setup to actually take these little dudes on properly. Since that's an aggravating factor I decided to treat it as a CR 5 since the group is going to need to weather some damage with no chance to retaliate.
So, how does CR math work on the party? According the DMG it's THE EXACT SAME. The bracket for increasing CR at this number spans from 3-6. The difference being a 6-person party downgrades the difficulty of a given CR. Yes, the same CR has different difficulties. The same CR fight can be deadly, hard, easy, etc. depending on the party. The only thing adding 2 PCs does in this situation is reduce the difficulty from hard to average. You're still going to see PCs drop because level 2 characters can easily be 2 shotted by CR 1/4 goblins. Therefore the level of tension doesn't change even though the party will have a slightly easier time in dealing with the threat.
The same sense of relative difficulty is true regardless of level. For the smaller encounters where you have one or two lieutenants instead of a bunch of squishy mooks you might throw in a couple more fodder or change the terrain slightly just to slow down the party. That's it. And guess what? GMs do this on the fly EVERY SINGLE DAY with ease. You can't tell me a professional game company with hundreds of employees on this single product will have any slowdown. I just don't buy it.
