They are several reasons on why the Surface Pro 1, 2, 3 (and many other systems) can boot Windows 8 so quickly
-> System Performance. The CPU and memory of the Surface Pro 3 (and 2 and 1 and many other systems, but I'll mention 3, as this is what is being talked about here) is far more powerful than the iPad. ARM architecture, which is what the Apple iPad is based on, is designed for low power consumption, not high performance. Yes, it is incredible how companies are able to push the architecture in providing great performance, same with Intel how it is able to take its own current x86 architecture and make it so power efficient.
-> Storage speed. SSDs vary in speed greatly. Doesn't mean that the system has an SSD that the system is automatically faster than age old floppy drive, if you know what those are. I have experience systems with so called "SSDs" inside, which made me wonder if its not a 5400RPM HDD inside instead, despite the fast CPU (Core i5 series), and decent amount of memory. Some manufactures for some models (low end) tend to follow a check list: "People want to see 128GB SSD.. ok lets put a 128GB SSD, but the slowest one money can buy as we need to keep the price low, and that will boost sales over the competitor, check!... ", and can do it for other hardware as well.
-> Microsoft talent. Say what you will, Microsoft has very talented and skilled people. That is a fact. They know how to optimize things, and continuously do research on how to make things better. Sometimes the improvement isn't as big or visible due that it is being offseted by newer features, but it is there.
-> Modern Windows systems uses UEFI technology, and Windows 8 fully supports it, allowing to make the OS boot faster. I am not sure what Apple iPad uses, and if it does indeed uses UEFI, to what extend.
-> Windows 8 uses hybrid boot. It hibernates part of the OS when you shutdown (part that doesn't really change, or isn't expected to change at any moment), making it boot quicker. Pretty smart trick.