I have already created more than ones zebedee's instances on server but performance seems not improved dramatically.
And is it more acceptable with one client?
What Firebird server version? (Firebird 3/4 and higher have significant improvements when dealing with high latency networks, like the internet).
It's also possible that compressing too much will use too much cpu. Then you need to tweak the settings a bit.
(Maybe even eliminating the compression completely if you're using the latest Firebird with optimized protocol.)