I dont know how you calculate that, but for my phone I get the payoff (given a 30% size reduction) to be positive below anywhere from 30-43Mbps.
When on celular, even 5g, I am pretty often below that.
When on celular, even 5g, I am pretty often below that.
Taking bike.jpg and bike-repackaged.jxl in that demo, on my Pixel 10 Pro in Firefox Nightly, the size and decoding times are:
bike.jpg - 147 kB - 10ms
bike-repackaged.jxl - 126 kB - 33ms
So, I save 21 kB, but pay 23ms.
That extra 21 kB isn't part of a new request, so when it comes to download performance, we're talking about an active response. At 3g speeds, in 23 ms you can download somewhere between 20-118 kB, so at best, it evens out.
If it's a lower end device, the decode delta increases. If it's a faster connection, the 21 kB saving matters less. If the image was cached, then I'm just paying the decode overhead.
All that to say, "I save 21 kB, but pay 23ms"? You'll save more if you stop sending your favourite web font over with your page and use the system fonts, and trim down your site's JS.