Why are zk-SNARKs so CPU intensive? - What is my CPU doing exactly and why does it have to work so hard when creating a shielded transaction? How many years till we see CPUs that can handle these proofs with speed? From what I understand JubJub will greatly reduce CPU usage so the need for faster CPU may not be needed.

Will JubJub be the key to getting shielded transactions on a Trezor device?

Thereâ€™s an article that refers to it as moon math, its serious stuff which I dont claim to fully understand. There arenâ€™t definites but thats the idea, that the update will reduce computational and memory constraints perhaps allowing for shielded transactions on mobile devices, its slated for next year, weâ€™re hoping early, check out the dev update category
Edit- concerning cpus, watched a nova about Internet Security once and it talked about basically a sort of quantum limitation on curcuit size. They talked about a design using microscopic crystals stacked vertically, like a skyscraper to use the intimate space around the cpu more efficiently, it was neat. Anyways the size and performance limitations go hand in hand, and theyâ€™ll get better still but the compounded advancment of technology will taper off, that is until we make the â€śQuantum Leapâ€ť!

Itâ€™s basically because you need a public key operation (e.g. at least one scalar multiplication on an elliptic curve) per arithmetic constraint in the circuit representing the statement to be proved.

Sapling will use more efficient curve arithmetic and a faster proving system, but the main improvement is from using an arithmetic circuit that is around 16 times smaller.