New shoes/slower times?

Just wondering if this is normal...I took my shoes to the running store because 4 of my toes had turned black and were sore. I wanted to make sure that I was wearing the right size. Turns out I wasn't...and I needed a lot more stability. I've been wearing the new shoes for about 3 weeks now...thing is, my average per mile has dropped about 15-20 seconds. Is this just part of the normal "need to get used to a stiffer shoe" thing?