Nice one! This would’ve also been impacting WAN Accelerator btw as WAN accelerator would've been talking to the gateway servers. Which, if you deployed a WAN accelerator at each site would’ve made the data flow:

Source Synology → VBR Gateway at source → WAN Accelerator at source → Wan accelerator at target → VBR Gateway at source (for target gateway though) → Synology at target.

So you’d have thrown traffic over the wan to the target wan accelerator, then back across the link to the vbr gateway in your source site, then back across the link again to talk to the synology at target!

We’ve certainly made things more efficient here!

Yes, now its great. Thank you really much.

I just have one more question. 
Can I configure server to be gateway and also WAN Accelerator or I need to configure 2 servers?

I think that configuring WAN Accelerator should give better performances now. 

If you do this, then definitely use High bandwidth mode based on the bandwidth metrics you provided, as a guidance:

<=100Mbps = Low Bandwidth mode

100Mbps< Bandwidth >=1Gbps = High Bandwidth Mode

>1Gbps = Direct

As long as your servers have sufficient resources to run the multiple roles, this should be fine to use gateway & wan accelerator on the same server.

Great,

I really appreciate your help today. I learned a lot. 
Thank you
 

Super helpful thread.  Thanks Michael, and thanks OP for the super-fast and informational responses getting to the root of the issue.