I' m also having this.
I have a fresh install of pi-hole, Ubuntu 20.04.2 LTS virtual server with 6GB RAM and 8 CPU cores 100G SSD.
All devices on the network are running really slow.
Local nslookup is instant.
google.. never makes it.. times out.
░▒▓ ~ nslookup pi.hole ▓▒░
░▒▓ ~ nslookup google.com ▓▒░
DNS is running on my Mikrotik Router not jute Pi-Hole