This issue can be caused by many things however if you are using a recent version of linux you may want to try to the following links.
Diagnostics of the issue
https://www.math.tamu.edu/~comech/tools/linux-slow-dns-lookup/
Details on the issue
https://udrepper.livejournal.com/20948.html
How resolv.conf works
https://www.man7.org/linux/man-pages/man5/resolv.conf.5.html