I can confirm that rclone does backs-off, as expected.
2018/05/15 11:09:46 INFO : Starting HTTP transaction limiter: max 0.5 transactions/s with burst 1
2018/05/15 11:09:47 DEBUG : pacer: Rate limited, increasing sleep to 20ms
2018/05/15 11:09:47 DEBUG : pacer: low level retry 1/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:09:48 DEBUG : pacer: Rate limited, increasing sleep to 40ms
2018/05/15 11:09:48 DEBUG : pacer: low level retry 2/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:09:50 DEBUG : pacer: Rate limited, increasing sleep to 80ms
2018/05/15 11:09:50 DEBUG : pacer: low level retry 3/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:09:52 DEBUG : pacer: Rate limited, increasing sleep to 160ms
2018/05/15 11:09:52 DEBUG : pacer: low level retry 4/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:09:54 DEBUG : pacer: Rate limited, increasing sleep to 320ms
2018/05/15 11:09:54 DEBUG : pacer: low level retry 5/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:09:56 DEBUG : pacer: Rate limited, increasing sleep to 640ms
2018/05/15 11:09:56 DEBUG : pacer: low level retry 6/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:09:58 DEBUG : pacer: Rate limited, increasing sleep to 1.28s
2018/05/15 11:09:58 DEBUG : pacer: low level retry 7/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:10:00 DEBUG : pacer: Rate limited, increasing sleep to 2s
2018/05/15 11:10:00 DEBUG : pacer: low level retry 8/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:10:02 DEBUG : pacer: low level retry 9/10 (error activityLimitReached: throttledRequest: The request has been throttled)
2018/05/15 11:10:04 DEBUG : pacer: low level retry 10/10 (error activityLimitReached: throttledRequest: The request has been throttled)