You really don't need to worry about Blueprint performance, its impact on overall performance is minimal. You'd have to do something pretty extreme for Blueprint performance even to be a factor.
Optimizing this by converting to C++ will be cool, but hardly necessary.
The main reason to use C++ with Blueprints is better flexibility. You can make complete games in Blueprint, but it will always be limited compared to what you can do with C++.
There is no point discussing which is better though, ideally you know what you need for your game and then pick the right tool for the job. Unreal Engine is much heavier and that makes it less suitable for certain game projects. I would pick Construct over Unreal for 2D prototyping and for 2D games which should be as accessible as possible (e.g. web based and light weight).
HTML5 games have a very interesting future and while Unreal and Unity can convert to it, Construct uses it natively. That does make it a lot easier for example to integrate with other web based frameworks.
I also appreciate that Construct loads up super fast on any laptop, which makes it ideal for prototyping or working on smaller games on the go.
There is no one tool to rule them all, and I don't think there will ever be. The important thing is to be aware of the strengths and weaknesses and take the most possible advantage of it. That's why I find it pointless to ask for 3D or native exports in Construct, just like it would be pointless to ask for Unreal to be significantly simpler.