Speed VPN Site to Site have problems ?

I have two line Internet from two site.

Site 1 : Speed 300Mbps, Fortinet 80D

Site 2 : Speed 250Mbps, Fortinet 140D

Case 1 : I connect VPN Ipsec or PPTP with 2 line by Router Draytek : Speed copy file together is 20MB/s to 25MB/s

Case 2 : I connect VPN Ipsec with 2 line by 80D and Draytek : Speed copy file together is 2MB/s to 9MB/s

Case 3 : I connect VPN Ipsec with 2 line by 140D and Draytek : Speed copy file together is 2MB/s to 9MB/s

Case 4 : I connect VPN Ipsec with 2 line by 80D and 140D : Speed copy file together is 2MB/s to 9MB/s

And why case 2 3 4 if using any router Fortinet speed very slow?

i try change another line internet, disable/enable DTLS but not working.