It's a bit pointless to talk about the performance of a game built for a 48 hour coding competition, but since you brought it up...

1) What are you basing your judgement of perfomance on? Not the HD recorded video that had two of my cores pegged the whole time I hope - because that's not at all representative. ChromaShift runs at 60fps on every piece of hardware I own (including a macbook from about 4 years ago) and generally runs at < 2ms per frame. There's a bug for some chipsets that causes chrome and FF to render canvas very slowly when hardware acceleration is on - if you're actually running the game, trying turning it off.

2) Even if it were slow, the comment you're making would be about JavaScript and Canvas performance not CLJS. The performance intensive bits of the game all delegate to JS, just like the performance intensive bits of any Clojure app are typically wrapped Java.

  > But back in the nineties we'd enter 48 hours or 24 hours "demo" competition and come up with stuff incredibly better looking than that.

None of us are game designers or builders. I'm sorry it didn't live up to a professional's expectations for a 48 hour game.

   > And... Drumrolls: simply "iterating" over a seq made of a hundred elements in Clojure apparently takes more time than that. 

It's tens of thousands of elements and seqs aren't meant to be fast. You can easily just use an array when performance is critical.

   > But why oh why use a language that is very arguable not meant to write video games in (Clojure) to... write a game in it as an example of what can be done!? (or as an example of how powerful an IDE supposedly is!?)

The game was never used as an example of how powerful the IDE is. The game is just a game. The concepts behind the game happen to be similar to how LT was designed. Do you have any issues with the actual design of LT? This post has nothing to do with ChromaShift.

   > And not a single one of these game is using that "Component Entity System".

No one must build games on top of Unity and all the examples going back to the early 00's (like Dungeon Siege) must be a figment of my imagination. Entity systems are not new by any stretch of the imagination. [1]

   > So how are we, Clojure believers (I'm a big Clojure fan and investing lots of time and energy learning it), supposed to take any of this seriously?

Clojure(Script) is very fast, but it's not C and won't ever be. I've never run into a performance issue that I simply couldn't drop down to the platform and then wrap nicely to get native JVM or JS performance.

[1]: http://scottbilas.com/games/dungeon-siege

BTW, constructive criticism doesn't typically use words like "terrible" :)