I'm going to disagree slightly here and say its usually overrated. For things like wrestling, boxing, Judo, etc its really important. Those sports require quick reactions and responses, so you don't have time to think. BJJ also has this, but to a much lesser extent and there is no need to be super fast in Bjj. I can't think of any boxers wrestlers or judoka with slow reactions, but there are bjj players who I would say aren't particularly fast. I'd say 75% of what Roger Gracie or Gordon Ryan do has nothing to do with speed and they could easily go through a check list in their head of what to do next when performing a technique. Drilling can be important in the beginning when you don't know how to do a move, but after a certain point, which for some moves will come quickly, there will be diminishing returns.
Another thing to keep in mind is that everyone is limited by their knowledge of technique. If you don't know how to do an anklelock well, practicing it 1000 times won't help much. You'd be much better off learning more about the technique and then practicing it 10 times. For most bjj moves the feel and reactions of others is more important than the speed that muscle memory can give you.
There are of course moves in Bjj that could benefit from drilling, but they aren't often the ones that get drilled. Things like technical standups, bridges, inversions, etc IMO could benifit from being drilled more, but I think the gains from drilling armbars will run out fairly soon.