You are right, nowadays there is no difference. Modern BIOS are much better than old ones on this side.
I don't remember any of my PCs with boot-time motherboard images delaying boot in any meaningful way. The only thing those ever did as far as I can tell is hide POST/boot messages.
However, I do remember some GPUs from 20+ years ago having an obnoxious boot-time image that couldn't be disabled and THAT did lengthen boot time since the GPU was stalling boot during video BIOS initialization for several seconds to ensure the monitor managed to sync and warm up enough to show output before letting the computer boot.
One of my friends had me repair an old Samsung LCD last summer and it had a boot image of its own that took about three seconds to go away, which would be quite annoying if it had an LED backlight. It was a CCFL one though, and those do take a minute to warm up before brightness stabilizes.