I am on OpenWrt 21.02.0-rc1, I have 150 mbps connection, my ISP router giving 150-180 Mbps on lan and wifi (ac) both. but on openwrt (DIR-2640) lan giving 130-140 mbps and wifi (2.4) is last stuck on 35 mbps and wifi (ac) last 45 mbps.
tested the same with stock firmware, its working fine but not with openwrt
i have tried Routing/NAT Offloading option but no luck.
I have a dir-1960, which is the smaller (3x3) brother from the same D-Link router season as 26x0,
(My guess so far is that xx40 just has the US radio country limitations, and xx60 has the EU radio country limitations, with no other differences, but I could be wrong)
on a unused wifi channel 40, so far I get similar throughput as on my older dir-860L (also mediatek7621 based) towards a 2x2 AC client iOS device: between 400 and 500 mbit / sec. from my side, the mediatek driver feels rather flawless on the dir-1960.
hardware offloading was enabled, but I reach mostly the same throughput with offload disabled.
my 2.4 throughput is around 140-150mbit.
So far am using 21.02rc3 (and have no issues, except that I have to disable „ Disassociate On Low Acknowledgement“, but that is due to some unexpected disconnects of devices and is not due to throughput issues).
Everything else is out of the box, no weird customizations/tunings.
So I would say that 45mbit/sec is too low and the smaller EU brother 1960 can really do much more.
Maybe ensure that you do not accidently use a 20-channel width. Maybe also go for rc3 or rc4 for next tests.
FYI the DIR-1960-A1 and DIR-878-A1 tho sold as 3x3
both the units I have are 4x4
I have have disconnects & will try "Disassociate On Low Acknowledgement"
check byte 0x34 in your factory partition
44 = 4x4 33 = 3x3