Scp rsync lftp too slow

hi,non-native english here,i'll try my best to brief my problem.
purpose:download some files from a remote server to my router's usb thumb.
device:OpenWrt 19.07.4 Xiaomi Mi Router 3G
tried scp rsync lftp,terrible speed,about 30KB/s.

iperf3 on the router:
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.01   sec   210 KBytes   208 KBytes/sec    2   92.8 KBytes
[  5]   1.01-2.03   sec  2.42 MBytes  2421 KBytes/sec    0   1.41 MBytes
[  5]   2.03-3.01   sec  3.69 MBytes  3870 KBytes/sec    0   2.46 MBytes
[  5]   3.01-4.01   sec  4.91 MBytes  5026 KBytes/sec    0   2.46 MBytes
[  5]   4.01-5.01   sec  4.92 MBytes  5028 KBytes/sec    0   2.46 MBytes
[  5]   5.01-6.01   sec  3.68 MBytes  3774 KBytes/sec    0   2.46 MBytes
[  5]   6.01-7.01   sec  4.91 MBytes  5031 KBytes/sec    0   2.46 MBytes
[  5]   7.01-8.01   sec  4.91 MBytes  5032 KBytes/sec    0   2.46 MBytes
[  5]   8.01-9.07   sec  4.91 MBytes  4725 KBytes/sec    0   2.46 MBytes
[  5]   9.07-10.01  sec  3.69 MBytes  4034 KBytes/sec    0   2.46 MBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.01  sec  38.2 MBytes  3913 KBytes/sec    2             sender
[  5]   0.00-10.01  sec  38.2 MBytes  3913 KBytes/sec                  receiver

iperf3's result is what i expected.
scp on a pc(remote server to pc,router to pc):the results are ok.
any advice to speed it up?

what i have tried:
reset router < not work.
tweak net.ipv4.tcp_rmem net.ipv4.tcp_wmem < not work.