Question: Was asked this a while back and while surfing the net it jogged my memory. A user was experiencing pretty poor performance when using site to site VPN’s. This is going to show the age of the question as they were using FortiOS 5.0.5. Normally, this is because of a bug relating to NPU acceleration on the tunnel experiencing the degraded performance. You can disable NPU acceleration for said tunnel and you will usually resolve the issue. Use the commands below.
config vpn ipsec phase1-interface
edit <tunnel name>
set npu-offload disable
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