I use s5cmd, it’s a fast parallel s3 cli client: https://github.com/peak/s5cmd
Haven’t heard of rclone before so can’t compare yet but s5cmd is supposed to work against GCS and other s3 compatible apis.
s5cmd[0], written in Go, performs cp in parallel to/from s3 too. I've been using it for some time and it's plenty fast.
Excellent walkthrough, love boto. We’ve recently been using s5cmd which we’ve found is ridiculously faster than boto without any extra boto tricks.