Finally they are gonna test this. But i can confirm this. If my website loads at f.e 4 seconds. The moment i enable jetpack out of the box with nothing done besides just enabling it it loads avarage 2 seconds slower. I tested this many times and have seen this result in users websites also. This is why i never used it besides all the great features it has. So i am curious on the test results. I hope they confirm my findings and in the end resolve that issue as it is a nice plugin.