We have been seeing some performance variability with Chrome 55 where 2+ second gaps may show up in the middle of waterfalls. It doesn’t happen with Chrome 54 and appears to be fixed in Chrome 56. From everything I can tell, it is a real issue and not WPT-specific but it may cause unexpected variability in your performance results (particularly when testing from EC2 for some reason).
If you need more stable numbers before 56 comes out (still a few weeks), I have created AMI’s in all of the regions that are pinned to Chrome 54 (with Firefox, IE 11 and Safari):
us-east-1: ami-15a94b03
us-east-2: ami-8d7356e8
us-west-1: ami-e0722180
us-west-2: ami-a0ed50c0
ca-central-1: ami-36fe4352
eu-west-1: ami-a0d7fcd3
eu-central-1: ami-309e525f
eu-west-2: ami-839a90e7
ap-southeast-1: ami-3ca60d5f
ap-southeast-2: ami-62555101
ap-northeast-2: ami-f392439d
ap-northeast-1: ami-f7f98490
ap-south-1: ami-f696e099
sa-east-1: ami-b5a43ed9