Everything runs in stable 60fps on my end, I wouldn't ship something that I know is broken. But it's definitely demanding a lot of resources.
One of the biggest CPU and GPU bottlenecks is the custom skeletal animation system (re-computing the coordinates for every limb is pretty expensive and then there's dozens of draw calls that each require rebuilding the world matrix), so I'd recommend getting something like Sndir's Model Format instead if you can (which is free!). My skeletal animation system is a bit of a disappointment because of the performance issues and inherent limitations, so I mostly intend it to be a placeholder so the asset can have characters at all... GM's built-in 3D modelling support is very limited to begin with, and I have no experience with 3D modelling tools, but I know how to use paths and trigonometry so I stick to my guns :P
The second biggest resource hog is me using a sort-of-pillbox-style hitbox for the player, doing a lot of collision checks to make sure they work smoothly with all sorts of terrain... but the collision amount gets increased a lot because I essentially do one raycast along every axis to handle movement, so this balloons exponentially in performance requirements. Depending on level architecture, you could get away with a much simpler hitbox which would improve this.
If you're interested in the gritty details, here's the results of the profiling I did of the game's performance:
(The control draw event isn't expanded, but essentially the bulk of it is drawing character models)