Slow VPN Throughput after flashing OpenWrt

After flashing my router with OpenWRT I am noticing very slow speeds when I am connecting to VPN from my PC. I am connected using the wireless.

These are some tests that I have done so far to be sure that it is related to OpenWRT:

  • When connected to VPN from my PC after flashing OpenWRT on my router the speeds I get are 1Mbps/~0.2Mbps
  • Before flashing OpenWRT when connected to VPN the speeds were as expected 20Mbps/10Mbps
  • When connected to VPN using my mobile hotspot (same PC just using mobile internet) the speeds are as expected 20Mbps/10Mbps
  • When connected to the internet (wireless) without VPN my speeds are 283Mbps/50Mbps as expected.

My device is a Xiaomi Mi Router 3 Pro running OpenWrt 19.07.4 r11208-ce6496d796 / LuCI openwrt-19.07 branch git-20.333.25999-bf1b7d3

Someone suggested that it is related to MTU and that I should lower it?

Can you give more information about VPN service please?

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Tell me if you may need more specifics and I will try to give them with as much knowledge I have.

The VPN Concentrator is Fortigate.
The client is FortiClient for Windows 10 using SSL-VPN with a custom high port for connecting.

I did some other tests meanwhile with another PC and by connecting via Ethernet to the router; same thing: drop in upload and download speeds. If I use another router/hotspot from my mobile device everything turns to normal

Can you try to lower MTU?

For example set it to something like 1300-1400
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