Ipsec Fortigate & MikroTik

Hello All, 

i need your help.

file copy (Windows share) is very slow and not stable.

i have a Fortigate 90E on one side and a MikroTik Cloud edition ( serious gear on the other hand)

1. i have a site to site tunnel and when running IPERF with 8 threads inside the tunnel i can get about 50MB

2. if i run the same IPERF outside the tunnel with direct NAT to the machine i get around 80MB 

my speed isnt the issue.

my issue is stability and copying files

1. when i copy files within the site to site the speed moves between 1MB to 5MB and sometimes drops to 0 - very not stable

2. if i try to copy files outside the tunnel via direct NAT to the server - i get 5MB-10MB with no drops to 0

i already set the MSS of the tunnel to 1350 on both end.

please help me :)

thank you