Same problem on Cudy WR2100 with VHT-NSS 1 device (no problems with VHT-NSS 2) running openwrt 21.02.0 or today's snapshot (r17750-213ce1d837). Performance is good on an custom snapshot version provided by Cudy.

Results with: OpenWrt 21.02.0, r16279-5cc0535800
kmod-mac80211 - 5.4.143+5.10.42-1-1
kmod-mt76-connac - 5.4.143+2021-06-06-22b69033-4
kmod-mt76-core - 5.4.143+2021-06-06-22b69033-4
kmod-mt7603 - 5.4.143+2021-06-06-22b69033-4
kmod-mt7615-common - 5.4.143+2021-06-06-22b69033-4
kmod-mt7615-firmware - 5.4.143+2021-06-06-22b69033-4
kmod-mt7615e - 5.4.143+2021-06-06-22b69033-4
[   10.962470] mt7615e 0000:02:00.0: HW/SW Version: 0x8a108a10, Build Time: 20180518100604a
[   10.962470]
[   11.259141] mt7615e 0000:02:00.0: N9 Firmware Version: _reserved_, Build Time: 20200814163649
[   11.312816] mt7615e 0000:02:00.0: CR4 Firmware Version: _reserved_, Build Time: 20190121161307

With Samsung M30

8 x 19.5 MBit/s VHT-MCS 2 VHT-NSS 1
14 x 21.7 MBit/s VHT-MCS 2 short GI VHT-NSS 1
3 x 26.0 MBit/s VHT-MCS 3 VHT-NSS 1
9 x 28.9 MBit/s VHT-MCS 3 short GI VHT-NSS 1
5 x 39.0 MBit/s VHT-MCS 4 VHT-NSS 1
9 x 52.0 MBit/s VHT-MCS 5 VHT-NSS 1
2 x 57.8 MBit/s VHT-MCS 5 short GI VHT-NSS 1
4 x 65.0 MBit/s VHT-MCS 7 VHT-NSS 1
2 x 72.2 MBit/s VHT-MCS 7 short GI VHT-NSS 1
3 x 78.0 MBit/s VHT-MCS 8 VHT-NSS 1
3 x 86.7 MBit/s VHT-MCS 8 short GI VHT-NSS 1

With laptop Win10 w/ Intel(R) Wi-Fi 6 AX200 160MHz adapter:

1 x 360.0 MBit/s VHT-MCS 9 40MHz VHT-NSS 2
52 x 400.0 MBit/s VHT-MCS 9 40MHz short GI VHT-NSS 2

Results with: OpenWrt SNAPSHOT r17750-213ce1d837
kmod-mac80211 - 5.4.152+5.10.68-1-1
kmod-mt76-connac - 5.4.152+2021-10-12-83598c2e-4
kmod-mt76-core - 5.4.152+2021-10-12-83598c2e-4
kmod-mt7603 - 5.4.152+2021-10-12-83598c2e-4
kmod-mt7615-common - 5.4.152+2021-10-12-83598c2e-4
kmod-mt7615-firmware - 5.4.152+2021-10-12-83598c2e-4
kmod-mt7615e - 5.4.152+2021-10-12-83598c2e-4

3 x 6.5 MBit/s VHT-MCS 0 VHT-NSS 1
3 x 7.2 MBit/s VHT-MCS 0 short GI VHT-NSS 1
9 x 13.0 MBit/s VHT-MCS 1 VHT-NSS 1
2 x 14.4 MBit/s VHT-MCS 1 short GI VHT-NSS 1
13 x 19.5 MBit/s VHT-MCS 2 VHT-NSS 1
8 x 21.7 MBit/s VHT-MCS 2 short GI VHT-NSS 1
1 x 26.0 MBit/s VHT-MCS 3 VHT-NSS 1
6 x 28.9 MBit/s VHT-MCS 3 short GI VHT-NSS 1
5 x 39.0 MBit/s VHT-MCS 4 VHT-NSS 1
5 x 43.3 MBit/s VHT-MCS 4 short GI VHT-NSS 1
9 x 52.0 MBit/s VHT-MCS 5 VHT-NSS 1
5 x 57.8 MBit/s VHT-MCS 5 short GI VHT-NSS 1
3 x 58.5 MBit/s VHT-MCS 6 VHT-NSS 1
10 x 65.0 MBit/s VHT-MCS 6 short GI VHT-NSS 1
1 x 65.0 MBit/s VHT-MCS 7 VHT-NSS 1
4 x 72.2 MBit/s VHT-MCS 7 short GI VHT-NSS 1
5 x 78.0 MBit/s VHT-MCS 8 VHT-NSS 1
2 x 86.7 MBit/s VHT-MCS 8 short GI VHT-NSS 1

Results with: OpenWrt 21.02-SNAPSHOT r16092-03865de4da (provided by Cudy)
kmod-mt76-connac - 5.4.119+2021-05-15-9d736545-4
kmod-mt76-core - 5.4.119+2021-05-15-9d736545-4
kmod-mt7603 - 5.4.119+2021-05-15-9d736545-4
kmod-mt7615-common - 5.4.119+2021-05-15-9d736545-4
kmod-mt7615-firmware - 5.4.119+2021-05-15-9d736545-4
kmod-mt7615e - 5.4.119+2021-05-15-9d736545-4

From dmesg:
[   51.988923] mt7615e 0000:02:00.0: HW/SW Version: 0x8a108a10, Build Time: 20180518100604a
[   51.988923]
[   52.260826] mt7615e 0000:02:00.0: N9 Firmware Version: _reserved_, Build Time: 20200814163649
[   52.298172] mt7615e 0000:02:00.0: CR4 Firmware Version: _reserved_, Build Time: 20190121161307

1 x 58.5 MBit/s VHT-MCS 6 VHT-NSS 1
4 x 65.0 MBit/s VHT-MCS 6 short GI VHT-NSS 1
2 x 65.0 MBit/s VHT-MCS 7 VHT-NSS 1
7 x 72.2 MBit/s VHT-MCS 7 short GI VHT-NSS 1
6 x 78.0 MBit/s VHT-MCS 8 VHT-NSS 1
48 x 86.7 MBit/s VHT-MCS 8 short GI VHT-NSS 1

while true; do iw wlan1 station dump | grep -A13 <mac-addr> | grep "tx bitrate" ; sleep 1 ; done | tee -a MCS-rates.log

sort -u MCS-rates.log | while read line ; do count=$(grep "$line" MCS-rates.log | wc -l) ; echo "$count x $line" ; done