It might be down to kernel performance then
- kernel version in TOS5.x is not necessarily receiving performance improvement features from the mainline development
- OpenWrt patches certain kernel code, including networking stack, that again may not be backported to from Master to stable
- TOS also adds some patches to the kernel code in their Master branch that are not backported to stable
- useful kernel config flags might be unset
- toolchain settings for compiling the kernel
- useful kernel knobs (sysctl) might not be set to optimise high bandwidth throughput (sample https://serverfault.com/questions/357799/improving-tcp-performance-over-a-gigabit-network-with-lots-of-connections-and-hi/758350#758350)
In short, it would take some effort to analyse in depth, e.g.
- start with a custom kernel build, preferably from mainline source and compare performance
- turn kernel knobs and compare performance
Here is some thread Can't get 1Gbit on Mox Classic, albeit about the M it touches some aspects of the intricacies of the kernel’s network stack and traffic control management tuning and patches in OpenWrt related to network performance.