I started using Fermancap about a year ago, I have not noticed a difference in the rate of fermentation (measured periodically via a refractometer), however, as others have said, the fermentations do look slower in the sense of krausen.
On a side note, does anyone know if it expires? I usually use about 5 to 6 drops per 5.5 gallons, and generally that works just fine. I, for the first time, used WYeast 3787, the Trappist high gravity strain, and it was if I added no Fermencap. I had a serious blow off that probably lost half the total yeast population. I added 10 more drops, and it fowled the airlock again, and again after more drops, and even a 4th time. Now the batch is pretty much terminal, but good lord.