No no, look at it the other way round.
If a 6 HD creature is considered to be a level 3 character for purposes of gaining levels, it would IMHO level up too fast.
The 1 HD = 1 level isn't true powerwise, but it's handy to keep the NPC from levelling too fast.
For instance, one of the NPCs of Out of the Abyss is a 6 HD Spy NPC.
If we consider it as a level 6 character, it has 14000 xp and needs to reach 23000 xp to level up. This means it needs 9000 xp, which in turn means it won't level up until well after the party (of level 1 heroes) have reached level 5.
The player characters level five times, the NPC only levels once, which mainly gives it a seventh hit die. It could get a third sneak damage die, or possibly a bumped proficiency bonus, I don't have the specifics.
That is what I mean by slow.
And that is what my proposal is about.
Thoughts?