I never gave this a thought as I am running Pi-Hole on Raspian Stretch Lite so no GUI.
Am using one of my old Pi B devices, only two USB ports, now that is how old it is
Just checking that this is sufficiently up to the job, assuming as it is only really checking data that it should be fine although some web pages do take some time to load.
Ghostery (Firefox addon) reported that home.bt.com (entered as bt.com) took 52.7 seconds to load fully and https://news.sky.com/ (entered as sky.com/news) took 43.1 seconds.
Yes, until a few moths ago I even did all of the Pi-hole development myself on a Raspberry Pi B. Now I'm using a Pi v3 as compiling FTL on the v1 model was really awfully slow.
The long loading times you see aren't related to the computing power of your Pi-hole but they are caused by your browser which tries to download ads (most likely via HTTPS) and never get a response.
If you run the following command:
dig google.com
you will be shown the query time. If you try several times, you will see that the query time will be greatly reduced after the initial query due to the caching nature of our DNS resolver.
To attack the long loading times, please tell us what the following commands tell you: