Primates apart, the reason for exceptionally fast "boot" for ZX-Spectrum (and most of hobby/home/game-console computers from that time) was that software was stored in the ROM that CPU was able to execute directly from. Also hardware was much simpler and did not require arcane initialization for SDRAM [1] and such. So power-on initialization was literally something like "set stack pointer and few global variables; clear the screen; print prompt; done"
Nowadays, everything is on external media (external as in "outside of CPU's memory address space"), so executable code should be loaded into the RAM first (and likely assembled from 100s of DLLs or .so's in that process). And before that, you need to initialize at least 3 hardware components (SDRAM, mass storage controller and mass storage device itself; often this list is much longer).
That said, embedded systems booting in under 1 second were available 6 years ago [2]. Not many seem to be interested.
[1] http://electronics.stackexchange.com/questions/96831/sdram-i...