I was going to post my own thread, but I feel like this is the exact same thing I am experiencing as well. The pihole had been working perfectly fine with no issues for quite some time, but about 3-4 weeks ago I started seeing this same behavior and haven't been able to figure out what exactly is going on. I probably need to document things a little better, but I feel like it may be better after rebooting the pihole for a short period of time. If I remove the pihole I don't notice any slowness issues.
I have a google wifi mesh setup. The pihole is configured for DHCP and DNS, and I also have pivpn setup on the rapsberry pi with the pihole. Not really sure what else to do at this point but take the pihole offline because it makes the internet so frustrating to use when it takes > 10 seconds to load a webpage.