How to speed up testing a WSClient using the "Mock the web service" strategy

In our project, we do large use of WSClient to call external APIs.
As suggested here ,
we are starting an internal server, and testing the WSClient against the server.

The problem is that, for each test, we need to setup a new server, to serve a different resource

 server = Server.forRouter((components) -> RoutingDsl.fromComponents(components)
                .POST("/").routeTo(() -> ok().sendResource("resource1.json")

and this takes a few seconds each time (2-4 seconds), so the test suite is becoming sloooow.

Any suggestions to speed up the tests?