Delenne - I opened Kiwi Story in C3 with Chrome Canary and it only used around 300mb of memory. It might be worth checking your browser addons, some can add a lot of extra memory usage, especially ad blockers.
I deactivated all the extensions in Chrome on my Linux Mint and Windows 7 - memory usage is now about equal on both platforms: ~450mb with the Kiwi project loaded up in Linux, ~520mb in Windows.
I have the following extensions on both platforms: Adblock Plus, and the Google applications (Docs, Sheets, Slides).
Much better now than the ~800-900mb on Windows.
When I preview the Kiwi game, memory usages goes through the roof again: an additional ~200mb is used for the preview window in Chrome!!! (Linux and Windows).
My view is that Chrome is a memory hog, and needlessly gobbles up memory, and that is bad news for anyone building a web app with Chrome as a foundation.
Not an ideal situation but we are beating a dead horse here.
C3 runs in the browser, with all the benefits that brings, but also with it comes all the negatives. i.e Construct 3 won't be as fast in performance as Fusion 2.5-3 , GameMakerStudio 2, GameSalad, Stencyl, Unity.
The question one should ask what is fast enough for your specific game you are trying to make. I think for most it will be fine. I really hope so. I like Construct and got along with it better than the other applications. However bad performance is obviously a deal-breaker if your game doesn't run well on a platform even though you did optimize it etc.
I feel your are right: even the performance of a simple platform game (Kiwi) is very underwhelming on a i5 machine with 4gb. Needless to say Godot runs smooth even when in preview mode on the same machine. And I tested by far more complex projects in Godot on the i5 machine.
My advice for now: stay away from Construct 3 if your intention is to create action games that must also run smooth on lower spec'd machines. And it worries me that the performance of such a simple game like Kiwi is lacking on a i5 4gb spec.
Note: I could not test the exported version of Kiwi, because C3 does not allow me to export the game. I would be surprised if it made any difference, though. Ashley: maybe it is a good idea to allow export of the demo games, so users may get a better idea of published game performance? Or is a published version of Kiwi available online?