First, most parvi species and many of the parvi hybrids are slow. With the hybrids I find there are a few from each flask that grow rapidly, the rest are slow. So your perception of slow might not have anything to do with your humidity, it may be the nature of your plants.
* About Humidity: my parvis get by with low 35% humidity for about 4 months out of the year. It is not ideal, but they get by. During the late spring, summer and early fall my humidity comes up with the outside air coming in. Seldom does it get all the way up into the ideal range. But my parvis get by. Some clones of armeniacum and micranthum I have had for more than 15 years. They bloom occasionally. Most clones only bloom about every 2 to 3 years. At ideal humidity (70-85%) you will get better, faster and more robust root and foliage growth. But at those humidities I have trouble with the paint peeling off the walls. Not good for the house. So I accept the slower growth.
* Humidity trays are not terribly effective, BUT they help you group the plants together. A tight group of plants will form their own "micro-climate" especially when you pass the 50 plants all in a bunch point. So go ahead and try the humidity trays. Keep them clean, detritus build up will spread disease and insects.
Leo