Using server.get() or server.update() feature in Service Portal widget to make a background Ajax request to the server results in the requests to get queued up at the server and are executed serially one at a time.
This is the expected behavior of the server. All parallel requests made to the server have to be queued and executed serially.
Use angular's built-in $http to do the requests and those can be done in parallel but it is not supported by ServiceNow at that point. Please note that even if these requests are sent to the application layer and are queued, they will be executed serially one at a time. There is no way to change this behavior.