Drawing lines is faster than you think! The perfect-freehand algorithm that I wrote / that canva is using here does not use splines but it does recompute the entire line on every frame. It’s fine at 60fps (and also fine at 120fps) up to a few thousand points on somewhat modern hardware before the paths get too slow to render. The algo itself is just blazing fast, in part because it does not rely on splines (which are much more complex).

For an svg-style approach to ink (as opposed to a raster / dab-style brush) there’s no other option than recomputing the whole line each time. As a bonus, you can adjust the properties after the fact very easily. (You can try that at perfectfreehand.com.)