Slow Download Speed but Full upload speed on Separate Wifi zones

Personally i would not use separate zone at all, if you have a bigger setup. Move to Bridge to VLAN, which is essentially the same if you have VLAN switches. 

Looking at the size of your deployment, there should be VLANs in place? 

We have Fiber network/ OLT-ONT devices in our environment. Current Bridge to VLAN setup has issues with device roaming, while going from switch network to fiber network area. 

The separate zone configuration solves all roaming issues. Thats why we decided to go with such configuration.

You mean, you do not share the same VLAN across all locations? 

Because actually, a separate zone is a tunnel network to the firewall. A VLAN should be the same. It should tunnel all traffic to the appliance. 

We have VLAN across all locations. But implementing this causes issues with roaming, since the location has fiber as well as copper network. Implementing VLAN is not a solution in this scenario, as we tried it already.

This sound like a fundamental issue with your deployment. If you cannot roam within your VLANs, it looks like the Switches having issues to span the same VLAN across the deployment. 

But beside of this, about your separate zone issue, you should create a support case. 

This sound like a fundamental issue with your deployment. If you cannot roam within your VLANs, it looks like the Switches having issues to span the same VLAN across the deployment. 

But beside of this, about your separate zone issue, you should create a support case.