I was working on my Dad's Dell laptop this weekend, and no matter how quickly I spammed the correct key (F12 in this case) it would miss it and continue to a full boot about 3/4 times. I never figured out if it is just picky about timing, or if it had different types of reboots where some of them entering BIOS wasn't even an option.
Newer Dell laptops have a BIOS option to artificially delay the boot process by a configurable number of seconds to give you more time to enter the menu. Which should be proof enough that the default time window is an issue.
Mine has a large delay between when the keypress is registered and the menu actually shows up. But, the window for pressing the key itself is quite short. Also, if you spam the key too quickly, it will hang indefinitely instead of entering the menu necessitating a hard-reboot. Good times.
