I'm going to dissent here and say yes, but with some qualification.
There are three types of slow:
Slow Conceptualization
There are those who spend a long time on choosing their shot. These players take too long to analyze the table. To write them off as overthinking is silly, for many of them are just a bit deficient in the process of organizing their thoughts in evaluating the table. Many such players have gone on to be great players, with the great Ralf Souquet a good example.
Slow Pre-Shot Routine
Some, once they have made their shot choice, waste a lot of time before they get over the cue ball. Some of them go on to be great players, with the great Jeremy Jones a good example.
Slow Shot Execution
Others get over the cue ball and seem to take forever before they fire. These players usually pay a price for this habit, which often evidences uncertainty about the choice they have made.
I think a player should be willing to become a slower player if they need more time in their conceptualization. Over time, their ability to evaluate the table will speed up again as their table assessment skills improve.
I do not think a player should be willing to embrace a lethargic pre-shot routine or become super slow over the cue ball, because this, far too often, will prove an unbreakable habit that will alienate both fans and opponents.
Taking more time to plan is OK, but once the decision is made, get on with it. At least that's my philosophy.