Munchkin lead association is asynchronous, yes. This is expected behavior. Although it can appear synchronous in periods of low utilization, that's coincidental.
Where are you running the API call from? I hope you're not calling (or passthrough proxying) the API method directly from the browser.
I think you should describe your goals in detail (rather than just the current implementation)... immediately retrieving the Marketo ID _on the client_ is not a typical requirement.