This too summarizes pretty well how I feel. I'm very close myself to abandoning Construct because of this. After 3+ years of learning and excercising optimization tricks, I still can't feel confident that my game will run hard locked at a given framerate. This is soul crushing when making action packed music games that can't miss a beat.
Just wanted to say that, in the end I didn't abandon Construct because of jank issues. I'm actually more impressed with C2's general performance now after replicating my project in GMS2 I ported it over and stuck with it because:
1) My event code was getting bloated and hard to manage, and with some long-standing bugs I struggled to squash. It was in need of refactoring. So I thought, why not try and port it to GMS2 while I'm at it?
2) Apart from UWP and WiiU, C2 can't deploy to consoles. I know console may be a bit of a long shot anyway, but with GMS the possibility is there at least.
3) The way GMS works is a bit better suited for pixel art games, and for me at least it's faster and easier to handle large amounts of animations in GMS's built-in sprite editor. Huge reason for me to switch right there, as my game has literally thousands of animation frames!
Not gonna lie, it's nice to not have to worry about jank issues anymore. But that was a minor reason for my choice all in all. And C2 definitely has some advantages over GMS. I can see why the jank is extra concerning for your game tho, being rythm-based and all.