You can use the client test tool to attach to Cube (Follow->Follow...->Hook into active session->[select Cube in the drop-down]->click OK). When the visual overview is opened, client test tool will intercept all the communication between server and client.

Using the timestamps you can then investigate which request takes a long time to respond or which events are slow to come in. Added bonus to this, once the slow responder is found, you can immediately open the request/response to see which data is forwarded to the server and use this for further debugging (i.e. to manually execute the request on other servers/datasets).

If the requests get a response fast but there is a delay before the next request is sent, then the issue is most likely in the way the visual overview is built where processing/displaying some data takes a bit of time.