The wiki says a lava source adjacent to the block the netherwart is on causes it to grow faster. this has been working for me. I usually have a row of 20 soul sand next to a wall, and fill the blocks under the wall with lava.
They growth speed is fine in the Nether. But what you need to do is create some activities for yourself in the Nether. Farm ghasts with a bow, kill zombie pigmen for gold, make a castle, farm blaze, just find something to be doing while your waiting for them to grow. I also do all my potion crafting in the Nether because that adds a bit more time to it.