Network traffic through openfortivpn is very slow (100x)

[Geert Uytterhoeven]

Version: 1.17.1-1build1
Release: Ubuntu 22.04.3 LTS

Network traffic through the VPN tunnel is very slow.
With the current upstream version from github (commit bf94c2f07a96c035 ("Deal with `sni` the same way as `gateway_host`")), transfer speeds through the VPN tunnel are more than 100 times faster.
I am launching openfortivpn from a shell ("openfortivpn -c <config>").

I have tried different versions from newer Ubuntu releases:
  - 1.19.0-2 (openfortivpn_1.19.0-2_amd64.deb from lunar): slow
  - 1.20.5-1 (openfortivpn_1.20.5-1_amd64.deb from mantic): fast, but this version only works after a fresh reboot.
    Running it a second time, or after an upgrade from an older version, gives a non-working tunnel
    (after closing the tunnel, it reports: "Sent N bytes, received 0 bytes")
  - 1.21.0-2 (manual debuild, as openfortivpn_1.21.0-2_amd64.deb from noble needs a newer glibc): slow