Unable to run JSContext in background thread

Question

Here I'm initiating the JS call from swift and based on the results app UI gets updated. Since this method is time taking process it freezes the app for a few seconds and becomes normal. We can understand it's running in the main thread so blocked the UI. I have tried to run it in the background using the below code and it doesn't work.

DispatchQueue.global(qos: .background).async {                        
            let fn = engineJSContext?.objectForKeyedSubscript("jsMethodName")
            let val = fn?.call(withArguments: [params1, params2])
    }

As per the below document, we can run it from any thread, but I'm facing a problem on the background thread. Any help on this, please! https://developer.apple.com/documentation/javascriptcore/jsvirtualmachine