Thanks for your advice.
I've tried to recreate the issue into a simple example, but unfortunately I can't replicate the problem. I'd show you the project but I don't think I'm permitted to do so currently, sadly.
I'm not setting the useRAF to false, although when I tried out of curiosity, both easeljs and TweenMax seemed to be in sync, but that they both slowed down to less than a frame per second during the intensive bit!
I put in logs on both the onUpdate of a tween that runs throughout and on the update function of easel, and at the most intensive parts, there's definitely a dif