OMG the time to break down items does NOT improve as you level???
Yes and No
The higher you get, the faster you are able to "produce" essences on lower levels.
If you are going for the highest possible item, it takes you more or less the same time as on the levels before.
For example:
You are Rank 21, which gives you the possibility to mute LvL 20 magical Items - it'll take you XY minutes
You are Rank 31, which gives you the possibility to mute LvL 30 magical Items - it'll also take you XY minutes
If you are Rank 31 and want to mute LvL 20 magical items, it'll take you a lot less than it used to be on LvL 21
Hope that somewhat makes sense ^^