A bug related to item check time on unreachable hosts has been fixed. This bug was introduced in Zabbix version 3.4.9 or 4.0.0. Now the item check period is recalculated based on the UnavailableDelay value, not on the item update period. This can potentially change unreachable poller performance. For example, if item previously had a delay of 1h then now it will be checked based on the UnavailableDelay value, not every 1h as it was (due to bug). Therefore it is recommended to increase UnavailableDelay value to avoid possible unreachable poller overload.