I was just wondering how much computing power would you need for a single transaction. If we look into the future and think of all the small little sensors, they all would want to perform the PoW. Otherwise they would outsource it, but then pay fees again. How long would an Apple Watch need? How long would a Raspberry pi need?
A nonvalue transaction (sensors would most likely just pass data and not iota) is only a single transaction (attaching an address to tangle is also a nonzero transaction). My PC (with an i7 8th Gen) manages around 1,5 Transactions per second. If you want to send value, it takes at least 3 transactions through (this is called a bundle).
Update 12.08.2018: There are now FPGAs out with highly increased effiency. They are able to calculate the PoW for several transactions per second while requiring a minimum of energy.
How long it would take on other devices is yet unknown as we lack benchmarks of specific CPUs and GPUs. However, in the very near future, there will be very small trinary processors, made for this PoW work. These could be used in sensors to transmit data.
I don't know any technical details of an Apple Watch, a Raspberry PI would probably not take very long either.
If you have a lot of sensors, you could also simply host a small node where the sensors send their signed bundles to, the node does the PoW then.
PS: You might be interested this post too: What would an attacker have to pay per transaction in a large-scale spam attack?