Throughput Slow 50% when WAN > LAN

I need a help to solve a puzzle in my router. Any helps is welcome.

When traffic pass from WAN to LAN its drop the throughput around 50%.
I tried enable SQM or disable it and no changes.

Version :OpenWrt 18.06.1, r7258-5eb055306f
Platform : Raspberry Pi 2 B Broadcom BCM2836
Kernel : Linux LEDE 4.9.120

Speed Test from Router:

Speed Test from Desktop (The Issue):

Iperf from Router to Desktop:

Iperf from Desktop to Router: