You get an experience bonus fighting higher level opponents than you, so even if lower level opponents start giving less or no experience, you can come out ahead.
It is just structures (towers, ballistas, etc) that give fixed experience points in dragon form, not wyverns and demons, etc, or flying fortress ground opponents in human form. Quests do give fixed experience points.
I didn't do any rigorous testing in DKS, but see the D2:ED topic
xp points per level list? spoilers possible.
If you are 1 level below an opponent you get a 1/6 experience bonus, 2 levels gives an extra 1/3 and 3 or more gives an extra 1/2. If you are more than one level above your opponent, the experience drops off as the inverse square of the level difference.
In DKS at the end of the D2:ED section I was level 37.47. In D2:ED I finished at level 36.75. At least part of the difference was due to balancing changes in DKS. For example, in D2:ED I got to level 18 in the fjords before heading to Sentinel Island, while in DKS I made it to level 22. The difference in experience on those two levels from mindreading the island was still only 11.7% of the experience needed to reach level 38. There was also additional experience from quests in DKS, though not a lot (I never tried to figure out how much), according to a Larian comment on quest experience balancing to make it easier to level at the start of the game.
In D2:ED I delayed turning in some quests, but didn't really go out of my way trying to level. I used the Talisman of the North for quite awhile, and between about level 13 and the Hall of Echoes got Wisdom up to level 9 or 10, 4 or 5 of which was from equipment bonuses (equipping Rothman's Bow to turn in quests). Before the Hall of Echoes I boosted it to level 13, to see if I could get to level 37 before the end of the game, thinking the area might be larger than it was. I reloaded for some useless mindreads, but not all.
In DKS I specifically tried to do all the mindreads first, then clear opponents as much as possible and then turn in quests, to maximize the experience given. I didn't learn Wisdom, though did eventually collect jewellery with a combined +8 bonus. I deliberately mindread pretty much everyone.
Neglecting the balancing and island mindread differences between D2:ED and DKS, at best (assuming only a modest increase in quest and opponent experience levels) my efforts brought me from level 36 and 3/4 to 37 and 1/3.
It seems there is a slight benefit trying to maximize experience, but not enough to worry about if you need to turn in quests early to level, or simply wish to do side quests as you get them, etc. If I wanted a more exact idea of the difference I'd play DKS again doing side quests as early as possible; I can't see that happening for my next character, but I will not be as strict about delaying quests.
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