gsutil rsync too slow when uploading files to S3

I have a google cloud storage bucket (Sao pablo) and a backup bucket in S3 (virginia). The command I use is

gsutil -m rsync -r gs://**** s3://****

It was working fine with less than 200gb. After uploading more than 350gb, every time I run the command, google cloud takes some minutes to list files ("At source listing ..." ) but the destination S3 takes an hour to list 10000 files, so I need a day to sync files. I don't understand what happened because if some days ago it took 3 hours for example, after duplicating the space I hope about 6 hours not a day. Any idea?