I don't know if gear is indeed the issue, but as MiRai mentioned, questing is an option. You could do that to catch up, to see to what extent the gear is indeed affecting your performance, or simply to get to character levels where the gear upgrades slow down a little so you won't be as far behind any more as you might be at earlier levels.

If you haven't already, you may also want to grab a damage meter addon such as Recount to get an idea of how your dps output compares to other teams of similar level perhaps?

Something to keep in mind at lower levels especially is that different classes get different (types of) abilities at different levels. Classes that get their AoE early can easily shine in dungeons, while others are still struggling as they're limited to single-target spells and abilities.

Another thing that doesn't always get taken into account is that your performance can also be greatly dependant on how the rest of the group performs (or is able to perform). As an example, a class/spec that heavily makes use of DoTs will show better damage numbers the longer mobs are alive, as only then do those DoTs get a chance to get some ticks in. As another example, in the Cataclysm days when levelling a single rogue, I would almost always top the damage meters when mobs were alive long enough for me to use finishing moves with the combo points I built up. If mobs died very fast simply because the rest of the group was doing a lot of damage, I would hardly ever get a chance to use them and as a result, my own damage output suffered greatly and I'd usually find myself just above the healer instead.

When it comes to multiboxing, mobs that die fast can also affect performance simply because you have to do a lot of target switching, and there is always that delay between the moment you've selected a new target, and the moment your slave(s) are aware of it (Client tells the server, server tells the other clients). It's only a small window, but when mobs aren't alive for very long, those little delays before your slaves can start applying damage to a new target can definitely affect your performance as well.