nslookup time out after 2 seconds but ping ok and inteet connection slow

The configuration is a single Windows Server 2012 with AD and DNS installed. Each client user can login properly on AD with his DNS pointed on the DC server controller.

But the issue is a very slow inteet connection thru this DNS server. If a client PC is connected directly on the main zyxel router, inteet connection is ok and fast. But as soon as it goes thru the Windows server, the connection is slow. When a website is "pinged" on Windows server, it's ok, but nslookup with this same website address doesn't work. DNS settings and redirectors are all good.

enter image description here See the screenshots for more details. Any help would be very appreciate. Thanks.

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