Problems with speed are very common and the causes vary widely.
Anthonyqueen1985 wrote
Any idea how to improve this trouble?
As a test, place your client computer in the same LAN as your server and connect directly then test the speed. You should be able to max out the VPN speed (more or less). I get ~24Mbit/sec on a 54Mbit WLAN.
Further reading:
https://community.openvpn.net/openvpn/wiki/Gigabit_Networks_Linux