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Update - this problem is now solved. Thanks to all the learning from the incredibly useful inputs here and on the Manjaro forums I was able to take it up with the ISP and have my router changed. The issue appears to be related to how the firmware of my router at 192.168.9.1 was handling DNS requests made to 192.168.9.1. Once Windows got the DNS addresses it appears it would cache it in the DNS Resolver Cache (I believe) and therefore there was apparently no problem with Windows. However it appears that Linux does not do OS level DNS caching as described here https://stackoverflow.com/questions/11020027/dns-caching-in-linux 1 and therefore each ping request would, because it treated 192.168.9.1 as the DNS server, cause the router to exhibit that 20s hang. Change of router solved the problem.

Update - this problem is now solved. Thanks to all the learning from the incredibly useful inputs here I was able to take it up with the ISP and have my router changed. The issue was that the router at 192.168.9.1 was taking a long time to forward DNS requests to the DNS servers specified. Once Windows got the DNS addresses it would cache it in the DNS Resolver Cache (I believe) and therefore there was apparently no problem with Windows.However it appears that Linux does not do OS level DNS caching as described here  https://stackoverflow.com/questions/11020027/dns-caching-in-linux and therefore each ping request would cause the router to exhibit a 20s lookup delay. Change of router solved the problem.

Update - this problem is now solved. Thanks to all the learning from the incredibly useful inputs here and on the Manjaro forums I was able to take it up with the ISP and have my router changed. The issue appears to be related to how the firmware of my router at 192.168.9.1 was handling DNS requests made to 192.168.9.1. Once Windows got the DNS addresses it appears it would cache it in the DNS Resolver Cache (I believe) and therefore there was apparently no problem with Windows. However it appears that Linux does not do OS level DNS caching as described here  https://stackoverflow.com/questions/11020027/dns-caching-in-linux 1 and therefore each ping request would, because it treated 192.168.9.1 as the DNS server, cause the router to exhibit that 20s hang. Change of router solved the problem.

Update - this problem is now solved. Thanks to all the learning from the incredibly useful inputs here I was able to take it up with the ISP and have my router changed. The issue was that the router at 192.168.9.1 was taking a long time to forward DNS requests to the DNS servers specified. Once Windows got the DNS addresses it would cache it in the DNS Resolver Cache (I believe) and therefore there was apparently no problem with Windows.However it appears that Linux does not do OS level DNS caching as described here https://stackoverflow.com/questions/11020027/dns-caching-in-linux and therefore each ping request would cause the router to exhibit a 20s lookup delay. Change of router solved the problem.

Update - this problem is now solved. Thanks to all the learning from the incredibly useful inputs here I was able to take it up with the ISP and have my router changed. The issue was that the router at 192.168.9.1 was taking a long time to forward DNS requests to the DNS servers specified. Once Windows got the DNS addresses it would cache it in the DNS Resolver Cache (I believe) and therefore there was apparently no problem with Windows.However it appears that Linux does not do OS level DNS caching as described here https://stackoverflow.com/questions/11020027/dns-caching-in-linux and therefore each ping request would cause the router to exhibit a 20s lookup delay. Change of router solved the problem.