Mame emulates the original hardware. If the original game took X number of seconds, mame tries to match.
So the question revolves around how the original game tested. Was it a crc check on part or whole chips? Did it involve multiple reads and (for RAM) writes? Was there a timed pause between chips/tests? Or did the next chip/test start immediately after the prior? Or were they done in parallel? Add that older machines had slower RAM/ROM and slower CPUs, so they expected to run slower (bits/sec) than current hardware. And last but not least, these tests were done only on machine startup, so they didn't need to finish quickly: most machine were on all day so no player saw how long it took to start up.
edit: RayB beat me to it.