We did actually test on older machines too, but the machine didn’t make a huge difference in this case, because it was network requests making it slow not JS execution.
Aye, the network is also superb at work... I don't have stats but it won't surprise me if the typical user is on 4G/3G and half of those people will always browse to your site at just the time when they're in an area of poor connectivity (e.g. that may simply be how it is at their home).
