I was playing a bit with OpenWrt wireless config and as a result (at least that is what I thought) wireless performance (tested from W10) dropped from 600-700Mbps to 240-300Mbps.
Was trying to revert all the changes and to test different builds and wireless interface firmwares but without the improvement. I didn't check however Windows wired performance because I verified with iperf connection between router and access point is working fine.
Until I have tested iperf over wireless from non-windows client.
To cut long story short since one of the latest updates only two Windows 10 builds (18363.592 with Atheros AR8175 and 19551.1005 with Intel 82567LM) out of a bunch of different ones I am using are not having that problem. However I have tested those unaffected ones with wired connection only.
All others have network throughput limited to around 300Mbps no matter wirelles or wired.
That is not really OpenWrt related but might save some time and troubleshooting.
iperf3 seems not to be the best choice to test from Windows. Single instance is for me capped at around 400Mbps while SMB file transfer at the same computer is easily reaching 600Mbps over wireless.
Same with two parallel instances of iperf3.
Looks like also build 19041.1 with Intel AC 7265 is not having performance issues nor problems with ping.
Might be the issue is not related to specific build or NIC but rather to antivirus/firewall package.
Good old Microsoft!
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In that case probably McAfee is to blame 
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Delete that NOW