Instant wake up depends on having all the running programs and data in the RAM chips. In sleep mode, the RAM chips continue to receive power so the data is not lost. If the power is cut off, the data will be lost, as RAM technology requires a continuous supply of power to retain the stored data.
Windows also has "hibernate" where the data in RAM is copied to the disk in the expectation of a power loss (the user selected "Shut Down") The copy on disk is moved back to RAM when restarting after the power loss.
After unexpected power loss, the OS and programs have to start up rather than continue. That take more time. If every part of the OS has to restart from scratch (after an install or an upgrade), it can be a significant amount of time. Windows does cheat on this by keeping another image of RAM that is "freshly booted, but no programs started yet" and using that to resume after most power cuts.