I have noticed that one can often reach the same objective either by using Effects of Compositions.
Which is more convenient to use in terms of e.g. processor power, memory, etc?

Tbh, I love the concept of "divide and conquer", namely you do stuff on clip A, you do stuff on clip B and then you blend them with a composition effect rather than working a bit on A and a bit on B to get the same result.
Pity that the Composition section seems a bit left behind, in most of the case you don't even have keyframes or even you don't have any parameter to tweak (take for example Divide, Burn, Hue...).

i think that compositions are something OLD.. like single track editors, i started doing video editing on 2000 with a large use of compositions and effe cts like that .. but on last 7 years i switched to "only effects" this make the project much more "dynamic"... tracks can be moved with effects, multiple effects can be copied at the same time from a clip to a clip (fadings/zooms/color equalizations).. also "vintage" "curtain" effects or now too old a them was used only on 1980 1990 broadcasting TVs.....
Kdenlive is able to "adapt" to every user (anyone can find the best for productivity in kdenlive) but for what a can see there are TOO MUCH things at the same time... Effects/Compositions/mixes/proxies/transcodings... and it's an hard job to maintain everything working when there are changes on kdenlive "non native" sources (MLT... FFMPEG..FREIOR.... ETC.. ETC..) also "mixing" this lot of features can create bugs/issues... i think that compositions are left behind because they was the first (implemented) way to do somwthing on Kdenlive (and now the most older and difficult to use on multitrack projects)

Many thanks for you answer. That clarifies A LOT!
But then a question naturally arise: in the current status, is there anything that cane be done only with Compositions and not yet with only Effects?
If the answer is negative, it would be a great idea to remove all the Composition thing to have more neat interface or, in the meantime, to mark it as OLD in the interface.

So, one would just have Mix Clips for "transitions", namely Wipe and Dissolve, and Effects for all the rest. Not a bad interface I would say!

Effects VS compositions. Interesting

In early times i often worked on super compositions. Needed to make something nice to not end up with some fake weak s***. Often with zooming effect. Or moving horizontal or closer or back and such, to compose vids.
Nowaday's i often film steady, and let effects do the composition movements (zooming, panning).
To be honest (shhhttt, don't give away this secret ) i even use photo's as background which with moving effects really look like real video

With 4k/1080p video to end up in 720p end-result there's so much possible. Which often wasn't possible doing those afterwards with the old PAL and NTFS formats, why those really needed those compositions while filming. Or s***** results with square video images (resolution).

BTW, this goes for much more.
My music doesn't need that delicate instrument and amp settings recorded no more. The vocals and mic doesn't have to be perfect while recording. As effects afterwards do so much.
My photo's don't need that much composition and camera no absolute perfect settings no more. Photo editing does so much.
Still, composition is very important and can not be forgotten. Dig effects quickly aren't looking natural no more. You can't create diamonds from rubbish. You need something good to start with. But less important then in the early day's.

barzi wrote:But then a question naturally arise: in the current status, is there anything that cane be done only with Compositions and not yet with only Effects?

Yes. You can't for instance reach a huge movement from left to right (or reverse, with up and down on it) with an effect (without loosing nice and honest view).
You can't let a shot move 'around' something/somebody with an effect.
You can't crash a car (with damage) with digital effect when you shoot an action movie (ps, WARNING, DO NOT go shoot this in real life composition!!!! ).
And more.
So i have to confirm you that most regular basic effects are possible with effects. Yes A LOT is possible with digital effects. And still a lot not.

Even then, real compositions will quickly be much more beautiful, much more active, if you film them in composition. For instance real 'serious 3D twisting and turning' effects are very hard to accomplish with effects only, while filming them real-life is almost easy, and still more beautiful and active.
So some 'very nice' end-result complimenting composition effects still have to be actually filmed. Those will make the difference.

PS: I am not talking CGI. In CGI 'everything' is possible. But this question is not about that level effects, i think.

is there anything that cane be done only with Compositions and not yet with only Effects?

A clip from track 5 can affect a clip from track 1, for me that's more than enough to exist.

it would be a great idea to remove all the Composition thing to have more neat interface

Menu: View->Compositions and uncheck the box, now you don't need to look at it.

I cannot remove anything in kdenlive, this tools just need some improvement.