[WireGuard] heavy packet loss

[k at vodka.home.kg]

I'm trying to connect 2 endpoints with wireguard.
One endpoint has MTU of inet inteface 1500, another 1456.
To prevent packet fragmentation on inet interface i set up mtu 1383 for both wg interfaces. Checked that no frag occurs on inet interface in wireshark. Maximum packet data size is 1383-28 = 1355. Checked with no-fragment ping, packets pass well.

First I checked packet loss between inet IPs

iperf  -c x.x.x.x -u -b 25M -l 1400
0.0-10.3 sec  29.6 MBytes  24.2 Mbits/sec  16.073 ms  175/23090 (0.76%)

packet loss always within 0.5..1%. its normal
now I try to do the same with much less bandwidth between wireguard ips
on packet size <=1252 packet loss rate is 0.5..1%

iperf  -c 192.168.254.65 -w 500000 -u -b 1M -l 1252
0.0-10.0 sec  1.18 MBytes   991 Kbits/sec   0.659 ms    9/ 1000 (0.9%)

but starting from packet size 1253 packets loss greatly increases. its 6..10%

iperf  -c 192.168.254.65 -w 500000 -u -b 1M -l 1253
0.0-10.0 sec  1.12 MBytes   936 Kbits/sec   0.328 ms   64/  998 (6.4%)

Thats why TCP is incredibly slow. If I set up mss for TCP to not exceed packets size threshold TCP is fast.