Update: This issue has been resolved in daily snapshots since thisthis commit). It is included in OpenWrt 21:
If you have an internet connection which is significantly faster than 70 Mbps, you may observe the maximum internet to LAN throughput with OpenWrt 18/19 is lower compared to LEDE 17.01. The following commit caused the throughput to drop from 140+ Mbps to 70+ Mbps when SMP is enabled.
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=916e33fa1e14b97daf8c9bf07a1b36f9767db679
Only one instead of both cores is used to steer packets to the LAN. OpenWrt bug 2573
Some workarounds are described in this thread:
OpenWrt forum link to modified 18.06.x image which removes the above mentioned commit.
Software Flow Offloading can also be enabled in OpenWrt 19 or later, BUT it is not compatible with Smart Queue Management (SQM).
It is also worth testing with irqbalance running (after installing with “opkg install irqbalance”).
Have the speed issues been fixed in the latest version?