SQM will reduce performance btw (and I think may be 'slightly broken' - see PDF link below). I don't think hardware offloading works. Using PPPoE will also reduce performance.
Check packet steering is enabled in Network menu
OpenWrt 19 may be faster but it does not support MR8300.
MR8300 probably has similar ethernet performance to my EA6350v3 tested earlier this year as both use IPQ40xx SoC
from PDF at:
https://www.dropbox.com/s/2y1l3k2l1nfibk6/EA6350%20speedtest%20results.pdf?dl=0
See this topic for optimized build.
https://forum.openwrt.org/t/optimized-build-for-ipq40xx-devices/44125