Yep. Think about Medium like 3D Photoshop, in a way; when you are adding clay with a large brush it's filling a ton of grid cells. Exactly like running Photoshop and picking a graphical brush and jacking the size way up - it doesn't really keep up with you as you draw.
Though, since I just went and tried that to make sure I wasn't lying, I notice that Photoshop hides that pretty well by fitting your stroke to a curve so you kind of can't tell. It's more obvious in Medium since we just connect them with lines. In the future we'll probably do stroke smoothing and whatnot, which should help smooth that out.
That said, it shouldn't be harming your framerate as it does that; we do all sculpt operations on other threads so we can keep the framerate high.
Another thing to note - if you pinch-zoom your sculpt, it doesn't adjust resolution, so if you pinch-zoom your sculpt down really small, it's "higher res", if that makes sense - you get more voxels per inch. So if you've zoomed your sculpt down really small and your tool size is maxed out, it may visually look like you're not adding much clay, but you're adding just as much as if it were zoomed up huge. (Scaling layers down is one way to increase your sculpt res.)
I noticed this effect last night. I don't think it's actually affecting framerate per se, as in refresh is still a steady 90hz but the clay updates lag a bit which kinda looks like a framerate issue as new clay pops in with a slight delay.
Yeah, it definitely shouldn't hurt actual framerate - that's the whole point.
As for how quickly the clay placement speed drops off, think of it this way - when you make your tool twice as large (double the radius of the sphere or whatever) that actually hits 8x the voxels (since it's 2*2*2). We have optimizations so it shouldn't actually get 8x slower, but it certainly falls off fast.
Stamp perf I think is still less than the primitives perf (sphere/cube/capsule) for a given tool size too, since it's basically rendering one voxel field into another.