Patience
How fast a plant grows is actually a combination of factors....
First off, if it is a plant you pay a lot of attention to and want to see grow, it will take literally forever for it to make any growth at all!
Also, time is not a constant as we believe it is, so to measure it as such is not a good way to decide if a plant grows fast or not. You can compare Neps in general to other plants, however a lot of how fast they actually grow is dependent on the conditions you are able to give it. A Nep can be a really fast grower under the right conditions, but can go into suspended animation and stop growth altogether if the conditions are off, so your real focus should be to give them the best possible environment, as anything less can slow them down dramatically.
Personally, if you like Neps, try them. Some people here get great growth, but their plants refuse to pitcher. In that case, what's the point? Might as well grow a coleus or something! If however, you have patience and other plants and hobbies to use up more of your time, then your plants will grow pretty fast, especially if you don't have room for a large plant!
An expensive plant will likely grow slower than a common cheap one, which is one of the reasons why it is common and cheap in the first place. (Which is a bit more factual then saying that the plant in any collection most likely to die, will be the one you paid the most for!)
If you are still concerned about having a Nep that grows fast, realize that as I mentioned already, it depends a LOT on the conditions you have it in. It is hard for me to suggest a specific plant to you, as your environment is likely very different than mine. Also, different Neps will respond quite differently to your unique environment. So if one type doesn't do well, it doesn't mean all Neps will. Even reading about them and their generalized environmental needs (highland, lowland, intermediate) won't be the same in terms of growth for every plant in each category.
Besides, if you think about it, it really isn't so much about how fast a plant grows, it is how GOOD it grows! If a nep of mine got one new leaf every 6 months, who cares just so long as the pitchers last a long time, it pitchers well, is very healthy and looks fantastic in the process!
I used to have a Nep that grew a number of yards long in a years time, however it didn't always pitcher and it was a real hassle to have to prune it back... it got unruly and messy looking rather quickly, and I would have rather had a slow growing Villosa, so long as it grew nice large and gorgeous pitchers on it! Now that is a plant worth growing!
Someone else even mentioned about how a pot full of normal Sphagnum moss nicely growing looks better then a pot full of Neps, when they aren't pitchering or doing anything other than looking like any other "green house plant"!
Think about it a bit. In my opinion, it is not how FAST it grows; it is HOW it grows, that counts!
I know this isn't the "opinion/advice" you were hoping for, but having grown CPs and other plants for as long as I have, I now value things differently, and see things in a vastly different way than I did when I was just beginning.
Growing Neps or other plants may be frustrating or difficult, but like any other plant, the rewards are worth it for those who love growing them. We are their caretakers, and we thus have a responsibility to give them the environment they want and need.
When we do our job well, we are rewarded by their thriving. And there is a vast difference between surviving and thriving... yet so many people are content to have plants that are just surviving, as it is a lot less work. Thus they never know the real joy of putting in the dedication and work necessary to get their plants to truly thrive. It may be more work, but ask anyone who does this and they will tell you, its absolutely worth it!
Again, good luck to all who want to grow plants they don't currently have, and don't give up unless you know you won't (or can't) give them the care they need. And even if you don't have the time, space or patience for them now, doesn't mean you won't in the future. Growing challenging plants can be one of the best experiences life has to offer!
Good Luck and most of all, Good Growing!