I've tried Decker on a few e-ink devices, including a Kindle's browser, and animations suffer of course, and drawing is also very laggy, to the point that trying to draw a circle results in a triangle getting drawn. I didn't explore performance much, but I could handwrite with some care and add widgets and it wasn't that bad.
Actually much worse than those devices is a 1Ghz RISCV device, the ClockworkPI. All animations are amazingly slow: going from the first card to the second card in the demo deck takes 19 seconds due to the sliding animation. Even opening a menu and moving the mouse down it has Decker struggling to keep up with inverting the colors of the menu item hovered over. But, it does work! It's slow, but it gets there! The main issue here is probably that this is a single-core machine and Decker uses all of it, a case where optimizing SDL refresh rates a bit might really help.
But if you mean a 15-year-old consumer laptop that was good at the time, Decker should be fine on that. Lightly retro hardware might even work a little better than the most recent hardware because you'll get features like Tracing Mode that worked on X but have been broken by Wayland. Decker itself should be fine, and I think you can track the additional burden that you add with the script profiler.