I thought quite much about how to express the issue in as few words as possible and at the same time to say the essence:

Which of the engines -- and I'm specially aiming at UDK -- offers in the best way, the possibility of making the simplest game with a corresponding low FPS?

This question I think reveals the best engine, besides the performance. Very tricky as some engines are not versatile and can't work with graphics below a threshold. I'm not implying anything about a certain engine; I just want to know more. Let's say we want to make a 2D game with no shaders, no nothing. Like a pacman or snake. Which of the engines is capable of making it and of those which can, which one yields the lowest FPS?

"Let's say we want to make a 2D game with no shaders, no nothing. Like a pacman or snake"
Then use Flash , it's the best option for that type of game.

For a 3d game with no shaders and older type graphics or technology , then use GameStudio or Unity(though they do support shaders as well, but they're easier to use and less demanding of end user PCs).

If you want to make the next block buster game with top of line graphics and want to use the best and most powerful engine you can get your hands on , then use UDK.

UDK is powered by Unreal 3 Engine , UE3 was use to make Unreal Tournament 3, Gears of War , Mortal Kombat vs DC Universe , Tom Clancy games , ect...
It's a powerful engine that used to cost three quarter of a million dollars to license , now at our hands. But if your looking to make corny pac man clones , then stick to something like flash , cause even GameStudio is way too powerful for that.

Excuse me but you got it wrong. It's not I want to make a pacman but it's the degree of freedom the engine allows. It's about granting access to the low levels of making a game. It's pretty easy to make a good looking, complex game but it comes with a cost. My music professor used to say: "Professionalism doesn't reside in singing correctly; it begins from using nuances". Well, maybe because I had some colleagues singing like soccer supporters... but anyway, think about making a game as building:

Different construction sites come with different size prefabs. Now it depends on what you want to build -- a hut or a skyscraper. I want to build a mansion. So I'd be glad to also have other prefabs than whole floors. But maybe not bricks.

I'm not against UDK, I haven't tried it yet but I planned to this week. It may be the only engine I'll use but I like having the control that you have in A7 (aside from frame rate). If UDK offers the same control degree (which I currently don't have a clue), believe me, I won't think twice.

I download and installed UDK last night..impressive, really impressive!

There are some very easy to follow video tutorials at their website. After you saw these, you will find it even more impressive.

Some tools are not very easy to get from the first glance, but after following a tutorial it makes lot of sense and reveals lots of power.

But I also found out that Unity is close behind them in terms of these tools.

Has anyone here tried anything serious with Unreal Kismet? I'm interested in this mainly because I am not a coder.

I tried using UDK on my current laptop, why too demanding for this hardware. Once I get my new desktop I'll start messing with the UDK. I plan on just messing around and making some simple shooters like I did with the A5 temps, only these templates will be much, much better (no offense to Doug)