I am a full time Construct 2 freelancer and I've completed many projects for browser, desktop and mobiles. Apart of making games from scratch I also had many requests from my customers to finish/optimize their game.
After several years with C2 I can clearly say that the most common performance issue with C2 is not a GPU but the C2 developers (or I should rather say their code). Of course GPU, CPU, your machine and C2 itself also takes some part in the performance issues but the majority of issues I saw was the C2 code (I mean the events of C2 devs projects).
C2 is pretty much like JavaScript or PHP or any other language that is not very strict from architecture perspective. What I mean is that it is very easy to make a simple things with C2 (like in JS for instance) but it is really a big challenge to make a big projects the right way.
The problem is (as mentioned above) the architecture freedom. So while making a big game people tend to use the same coding approach as for small games and in the end they are lost in the messy code which is totally unoptimized as you don't really need to care for optimization in small projects.
With big projects it's different. There's much more code/events so much more operations for CPU and therefore you must use the best coding practices so each feature uses the least CPU as possible. And next to that you need to keep your code architecture very organized to not get lost in your big project (and that is also not very easy with C2).
It also depends on what you call a big project. I worked with projects made of several thousands of events and after optimization they worked well on majority of modern desktop devices. Still the bigger project, the bigger challenge.
So to wrap up, in my opinion it is possible to make a big project (not sure about a huge one) in C2 in a way to make it work very well, but it's not an easy task. Definietely not a task for a C2 newbie. It needs experience and deep understanding of how the memory managament works in C2, which conditions are the real triggers and which are fake triggers, what takes the most CPU, how to make workarounds for high CPU intensive parts etc. Without all this knowledge you will probably fail in making a big game in C2 as C2 is simply difficult for such projects.
From the other hand if you pick Unity or any other engine you also need to learn how to use it properly.
EDIT: Just a clarification as I received some PMs. Please note that I'm not saying that performance issues are not related to the C2 itself at all, I'm just saying that the most common issues comes from the bad coding.
I agree 100% with all of this...