TIE's impact should be minimal since it adds no resources at all to the game. I can't give you numbers, but I can describe what I've seen in the sixteen hours I've played it:
The number of people on the streets in cities is moderately increased, because there are more (and more types of) guards, as well as some adventurers of many types who wander about. A very small number of objects have been placed in the world, at least so far as I have noticed, which all have important functions: Secret entrances/exits, sewer accesses, hideouts for the player, etc. As long as you don't start fights in cities the impact of these city changes should be small. The author showed restraint and didn't overdo it with a bunch of unnecessary folks on the streets.
The number of people inside taverns and guilds is greatly increased. I often walk in and find six different people all chatting it up, and sometimes two or three in side rooms, and taverns are really noisy places now. Some items have been added to both guilds and castles. There is probably an associated performance drop in these public places, but it has not been noticeable for me.
In private cells, homes and the like, there should be no performance change.
There are more people on the roads, but fewer monsters. There are fewer monsters in external cells everywhere, in fact. Out-of-doors, this probably amounts to a performance increase in most areas.
I can't speak to the content of dungeons, as my level two character has backed out of almost every one upon seeing an ogre or ghost in the foyer.
The maker of TIE made a few efforts to improve performance by (for example) getting rid of footstep sound files (or, replacing them with zero-length files, rather) for most of the smaller animals, such as deer, small boars, and feral dogs. This has meant, in practice, that packs of feral dogs have sneaked up on me twice, to my chagrin.

Hope this helps,
Asterai