Stereo camera Calibrator App runs very slow.

Hello, I'm using Stereo camera Calibrator App in Matlab 2014b. Initially, this app works fine and it took less time (in a minute I was able to do the calibration with 50 stereo images). I calibrated my stereo cameras many times and I deleted some stereo images which cause error and then recalibrate it. All this procedure took less time but from 2-3 days this App is taking 15-20 minutes for one calibration. I don't know why? can somebody explain it to me?