Please advise why PR_SYS_DELAYED_QUEUE is exclusively locked for long time, tough does not have custom queue processor

Alert log

GMT*8*PEGA0005*604496*500*8ab77511a7ddcc92696d0b94950c383b*NA*NA*BAZBZSNHVE0XRRE8ZTO1PHCGEND8AYLKEA*System*PegaSample*null*c5f8d11e1c6712b822be592273fd678f*N*1*BAZBZSNHVE0XRRE8ZTO1PHCGEND8AYLKEA*492*JOBSCHEDULER_THREAD_0*STANDARD*com.pega.pegarules.data.internal.store.DataStorePreparedStatement*NA*NA*Rule-Obj-Activity:pzProcessDelayedQPItems*RULE-ASYNC-QUEUEPROCESSOR PZPROCESSDELAYEDQPITEMS #20190806T061337.978 GMT Step: 1 Circum: 0*NA*****pxRDBIOElapsed=604.50;pxRDBIOCount=2;pxTotalReqCPU=0.00;pxRDBRowWithoutStreamCount=2;pxRulesUsed=1;pxTotalReqTime=604.50;pxActivityCount=1;pxRulesExecuted=1;pxAlertCount=1;pxRDBWithoutStreamCount=1;*Rule-Async-QueueProcessor*NA*Rule-Async-QueueProcessor*NA*Java;RULE-OBJ-ACTIVITY RULE-ASYNC-QUEUEPROCESSOR PZPROCESSDELAYEDQPITEMS #20190806T061337.978 GMT Step: 1 Circum: 0;doActivity Rule-Obj-Activity:pzProcessDelayedQPItems;initial Executable;0 additional frames in stack;**Database operation took more than the threshold of 500 ms: 604,496 ms    SQL: UPDATE LIPEGA_DATA.pr_sys_delayed_queue "PC0" SET "PYITEMSTATUS" = ?  WHERE  ( "PC0"."PYITEMSTATUS" = ?  AND "PC0"."PZTIMEFORPROCESSING" <= ?  )*