Suspend mode saves state of your OS into RAM and stops to power almost all of the components of your PC, except RAM(where OS state is saved), hardware clock, some ACPI peripherals. As OS is saved in the RAM of your computer, it takes almost no time to boot your system again, after powering machine on.
Hibernate saves state of your OS to the disk and powers off your system completely. That’s why your system will take regular time to boot, but it will boot into the state, in which you hibernated it. In Linux systems(and NixOS is not an exception), swap partition or swap file is used as a space on your disk, to save state of your system to.