I definitely think a fighter that drops to their knees deserves more time to defend than two seconds, and they usually do get it in male MMA. In most male MMA fights they'd get twice that at least.
And yeah it's completely normal for a ref to gauge the amount of damage that appears to be being inflicted from strikes when judging a stoppage. And in fact title fights usually get special consideration where the challenger is given extra time to recover because of the important of the situation.
I didn't feel like Ronda's little punches were the kind of intensity that warranted jumping in and calling a finish in literally two seconds.
A fight can be stopped by body shots the same way a fight can be stopped with strikes. When someone gets rocked/falls down from strikes, typically the ref gives it some time, waits to see if the person can defend themselves, if they stand back up or maybe roll over to their back to defend from guard, anything.
In this fight, there was 0 time. Herb gave her no time whatsoever to try and recover. She wasn't TKO'd or knocked out or unable to fight, she got hurt, dropped, and then started getting back up - the same way people get hurt, dropped, and get back up in countless other fights. If the fighter is completely conscious but falls from pain, you give that fighter some time to defend, make sure he/she is actually unable to continue. In this case, Sara was most certainly able to continue and it was an awful call.