The above idiom is usually said when we want to encourage a person to be consistent in whatever s/he is doing until s/he reaps the rewards. That is to say, we will tell the person to do things slowly but consistently and one day s/he will succeed in whatever s/he is doing. So, you can be as slow as a tortoise or as slow as a snail but if you are persistent, you can still win the race.
Is that ALWAYS true? Definitely ‘NO’.
That will not work when you are doing time-based activity or when you are pressed for time. In these activities, speed is very important besides quality of course. You’ll be working against the clock or you have to work with clocked speed. Taking an exam, taking part in a competition where there is a time limit, or submitting an assignment within a due date. If a person follows the idiom ‘Slow and steady wins the race’, then definitely s/he will lose out or even fail.
In other cases, ‘faster is better’ or the ‘early bird catches the worm’. So in this case, the snail with its slowness, will never get to eat any worm since it will only reach its destination in the evening. (I think snails are herbivores btw). In what cases can we attribute to this? Examples of such situations are getting a new iPhone, being early to take advantage of promotions or sales, or simply to get that particular ‘person’ that you admire so much. If you plan out your strategies slowly, somebody might just grab it first and you’ll be left biting your lips.