I just did a small test with a usb 3 drive copying ~20Gbytes of data over windows share/samba, and I get:
Read: 44MB (mbytes)
write: 32MB
I think the huge performance penalty you have is maybe because of the filesystem you use on your drive. I have my desk formatted as ext4… and I’m running vanilla openwrt. The SoC is not that slow when using native protocols (at least not 5-7MBps territory).