OpenVPN slow on AC-86U with Merlin firmware

[dr_lucas]

I noticed some of my openvpn clients perform significantly slower than the same clients on windows, even when completely disabling cipher.
I tried to compare the logs and noticed that the "Socket buffers" show significantly higher values than I have set on the remote (centos 7) openvpn 2.4 server and the router client.
Also, I see this line in the log:
OPTIONS IMPORT: adjusting link_mtu to 1624

This link_mtu seems way too high. On windows client I am getting
OPTIONS IMPORT: adjusting link_mtu to 1524

I SSH'ed the router and tested the openvpn on my configuration file directly, just to see what the log shows, and here it shows:
OPTIONS IMPORT: adjusting link_mtu to 1524
But still the "Socket buffers" are as high as through the Web UI connection.
Screenshot (left side is the windows client log):
https://lh3.googleusercontent.com/-...ACHMYCw/s0/TeamViewer_2018-05-15_17-11-51.png

How can this be adjusted?

P.S. My ASUS Merlin firmware is 384.5 (tho had the same issue on previous firmwares AFAICR)

On my server config I had set:
sndbuf 262144
rcvbuf 524288
push "sndbuf 262144"
push "rcvbuf 524288"

On the router client's custom config and windows client ovpn:
sndbuf 262144
rcvbuf 524288

 

[dr_lucas]

Does anyone know how to adjust link_mtu speed on the router, or where is it configured?