Slow VPN connection

I have an IPSEC VPN tunnel between a Fortinet 200D(with 5.4 firmware version) and a Fortinet 60E(with 5.4 firmware version).

The connexion a very slow (it is not like it's not working at all) , although the data link is 20Mb/s on an optical fibre support.

A test link to internet shows normal activity and speed.

Can you please help me .

Thanks.