Server side search issues

My setup as follows:

Zimbra NE 8.5.0 Patch 2 level
Zextras mobile licenced 2.0.2
Client: Blackberry Z30 OS 10.3.2
Activesync 14.1

Server side search causes high data+CPU usage on BB. Search delays a lot and returns zero (0) results (nothing found).

In comparison iPhone returns quickly all expected results without excessive data usage.

Please advise how to resolve this issue because server side search on BB os a vital tool in our organisation, and should work reliably as it does on iPhones

Regards