Well, i think problem is on client side. Entire cluster is a client cluster: if you have same request on every node then it will saturate client and application code. It is not a problem for most other DBs since there are no 5 second limit, but with FDB all transactions start to timeout since client is overloaded.
I suspect that this is mostly client, not our application code. I actually found that our application code could be very performant with FDB and it is usually client saturated before our application (like Bindings seems to be very slow for a simple operation).