I've found similar problem at Cancel transfer if speed drops below a certain threshold? - Help and Support - rclone forum
seems that my ISP have some sort of flow threshold ,thus a single tcp connection can never keep high transfer speed, its speed decay over time (only new connection have high speed transfer), but may never disconnect. rclone status shows such transfer state as 0/s , 1000h . since it not disconnect ,rclone won't retry so it hang forever. I can only interrupt whole rclone to make them work again. so I'm thinking if rclone can support a lowest threshold parameter, and retry at such condition ?