Reading your original post again, it seems you are using the gl.inet firmware.
As I mentioned in my reply, I’m using vanilla openwrt. While the gl.inet firmware is basically the QSDK, which itself is based on openwrt, the changes from that to current openwrt are substantial (4.4 vs 4.14 kernel, DTS-based, different USB drivers/ext4…etc). Could be any of those…
I didn’t expect that much of a performance discrepancy, maybe its a minor thing… but it could be anything.