One sign that your website isn’t meeting the needs of all your users is when Matthew Somerville gets sufficiently grumpy about it to do a proper version himself.
I first came across Matthew in the early 2000s when he got cross enough to build an accessible version of the Odeon website, allowing people whose needs Odeon had decided were not worth meeting to book films at their cinemas. You’d think Odeon would thank him, but no, they sent him nasty legal cease and desist letters. This wasn’t wise.
On the basis that it’s better to show rather than tell, here’s a side by side video of the two versions loading and rendering over 3G on a mobile.
Addendum: Matt Hobbs, who is the Head of Front end for the UK Government Digital Service, wrote a fantastic blog post this time last year explaining why GDS focus so much on performance. Contains useful stats.