Slow speed from WAN to LAN

I'm sorry for my poor English.
The WAN port of my router(domywifi DW33D, ar71xx nand) is linked to the local network of my school. I can download or upload files at 100mbps from FTP servers.
But if I flash the recent build of master branch to my router, the download speed is only around 10mbps while the upload speed is as usual. I find that the kernel of the last good build was 4.4.87. Newer builds doesn't work well. I also tried to switch the kernel to 4.9, but it didn't work either.
All builds of lede-17.01 branch work well on my router. And builds of both branch work well on another MT7620 router.

Is there any solutions?