Have you tried the current generation of iPad Pro? They claim a 9ms lag, and coupled with their technology for up-rezzing refresh rate and touch sensitivity when using the Pencil, it's quite a substantial improvement in experience.

That being said, the iPad Pro is clearly a high-end general-purpose device, with a price to match. The device that fits our pocketbook is always superior to the device that is amazing, but remains in a box in the manufacturer's shop because we can't justify the price.