CircleCI 2.0 will be switching file systems on Monday, December 11, 2017 from AUFS to BTRFS. After extensively testing performance we have found that any projects using heavy I/O operations should see a performance improvement without any action required. The underlying CPU is not changing and therefore your cache will not be impacted. We do not expect any projects to be negatively impacted by this change.
EDIT: CircleCI 2.0 will be switching file systems back from BTRFS to AUFS. We have not seen the performance gains we were hoping to see when fully rolled out in production. As a result, the change back to AUFS will be made tomorrow morning PST, Wednesday, December 20, 2017.