As 
newt pointed out, this benchmark actually shows Construct 3 has better performance than GMS2, at least on that benchmark.
Construct also lets you code in JavaScript, which has far more features than GML with loads of powerful programming syntax and tools that have been in development for decades. It's also an industry-standard language that you can get a job working in. GML is years and years behind JS in features and is only used by one tool, so you'd have to learn another language if you go anywhere else. Using JS from the start means you have a much more powerful language, and you're learning transferable skills.