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.