Thanks for the replies all.
@aliasmaximus - the network app is built into the Chrome browser (but Safari and Firefox both have equivalents). Usually something like Menu -> More Tools -> Developer Tools.
@john of MT - good point, I'm based in the UK. That might be a factor. Round trip to the US should still only be around 0.5s, so 5s feels a higher than I would expect.
My guess is that it's something to do with Guitar Tricks use of Cloudflare. But I'm really no expert. If it helps your developers diagnose, I seem to be hitting a California based IP for the request for dynamic content.