I'm facing the same issue.
The speedtests are fine, but browsing websites is really slow.
Chrome says it's trying to load local.adguard something, and it takes at least 15-20 seconds until webpages are loaded
I have Adguard installed both on my iPhone and mac.
When I'm using WiFi with Adguard on, everything works fine.
When I'm connecting to my iPhone's hotspot through WiFi, it works fine as well.
But whenever I connect a USB cable to my iPhone, I can't surf the web, unless I turn Adguard DNS...