The sync_clock method calls get_service which does a ListBuckets operation. If the IAM user does not have permissions to ListBuckets (as I assume would be common when granting bucket specific permissions to the credentials used for fog) it generates a ton of http 403's with AccessDenied (which can get logged in AWS Cloudtrail).
This is unnecessary overhead if the server's clocks are already synchronized
Add option to disable sync_clock calls: