SoftEther VPN has very slow download, while upload is high

I've installed SoftEther VPN Server on Ubuntu 16.0.4.1

VPN clients: iPhone, Windows 8 and another Ubuntu Server

Network Traffic Speed Test Tool:

without VPN:

• Download - 36.88 Mbps
• Upload - 4.93 Mbps

with VPN (L2TP or SSTP or SoftEteher VPN client with Local Bridge or SecureNAT):

• Download - 300.53 Kbps
• Upload - 4.79 Mbps

from VPN server to another server without VPN (same LAN, 1Gbps):

• Download - 0.95 Gbps
• Upload - 0.92 Gbps

from another server to VPN server via VPN:

• Download - 9.14 Mbps

• Upload - 905.33 Mbps

  ---

• Download -only - 88.18 Mbps

• Upload -only - 0.91 Gbps

Why Download is very low ?

Solution 1:

There was a problem with Local Bridge (eth0), so I changed it to Local Bridge(tap) + br0(tap + eth0) => issue is fixed

Solution 2:

This is how I did it (ubuntu 18.04/20.04):

(1) Set up a bridge (br0) using netplan config and add the NIC interface only to the bridge (in /etc/netplan).

(2) Add tap_softether to the bridge in start up script (in /etc/init.d).

brctl addif br0 tap_softether # adding tap device after it's up.

(3) Results: Download on openvpn went up from 1 Mbps (on physical bridge) to 100 Mbps (on tap bridge).