If you've got a speedy internet connection at home, but it seems slow, it's possible its' not the connection itself but the speed of your chosen DNS server.
To figure out if the DNS servers are part of the problem, check out namebench, a DNS server benchmarking app. namebench compares your existing DNS servers to a large list of other DNS servers, and shows you how they all perform.
When namebench launches, you'll see a window populated with your current DNS server addresses, and a few other settings you can modify:
Click Start, then go ahead and find something else to do for a while—the benchmarking process may take 15 minutes or more, depending on how many name servers it can see.
While the app still runs in High Sierra, you won't see any output. However, thanks to commenter Marc K., you can still see the output once it's done. Open Terminal and paste this command:
find /var/ -iname "namebench_*.html" 2>/dev/null | xargs open
This will open the results page in your browser—including the below-referenced eye candy—so you can compare the various DNS that were analyzed. Thanks Marc!
After namebench has done its thing, your browser will open showing a page with results, including some "eye candy" charts like this one...
The report may tell you your current DNS is the fastest, or it may have recommendations for other servers. Note: You may also see lots of scary messages about a DNS being hijacked; you really don't need to worry about these. (CHeck the FAQ for answers about hijacking, and anything else in the output).
namebench can also be built from source, so you could use it in Terminal, but I haven't tried that. It's not new code—the latest version (1.3.1) is from 2010. However, it seems to run fine in Sierra (and Mavericks before that). If you're curious about your DNS servers, I haven't found a better tool than namebench to investigate their performance.