When using setValue() on a reference field, be sure to include the display value with the value (sys_id). If you set the value without the display value, ServiceNow does a synchronous Ajax call to retrieve the display value for the record you specified. This extra round trip to the server can leave you at risk of performance issues.
More at https://developer.servicenow.com/dev.do#!/guides/tokyo/now-platform/tpb-guide/client_scripting_technical_best_practices