There is no reason to level slow at all, never in these kind of games. It doesn't matter that you are stronger than 99% of the player in your server who are at the same level. It doesn't matter if you are weaker than 99% of the player who are at a lower level. Like Kyutaru pointed out there are tons of benefits for leveling as fast as possible. Because in the end just by gaining a level, you gain power, you gain access to new team instances, more cultivation, awakenings etc. There is no need to keep it at your level, once you reach the max level, you will have time to max them out.

@Neconis that are very specific instances... We do not know when we get the update and while that sure has a huge advantage it's hard to say when exactly to stop doing plot missions. Especially if a new player who just started reads this, might gets confused. By the time he hits 70 we might already have level 90 patch.