The question is too broad , can you check/trace what queries are run that trigger that high cpu on the server side (maybe is garbage collection that is triggered )
Also for variables there are many articles how to tune them http://www.slideshare.net/ibsurgeon/resolving-firebird-performance-problems http://www.slideshare.net/ibsurgeon/resolving-firebird-performance-problems Try to take them one by one and check what they really do
