A SPA backed by rails is probably going to make at least 10 requests on page load. So in terms of actual traffic, 100k page loads during a peak hour. Assume a roughly linear peak increase/decrease and we've got roughly 1M page loads per day. 30M page loads per month.
How many websites have 30M (non-unique) page loads per month? After some rough scouting on Alexa ranks, I'd put the over-under at probably 10k US sites, and 50k worldwide. Assuming Alexa has 50M sites, then 0.1% of publicly facing sites serve that much traffic.
Rails is used often on private, internal sites and tooling. Those sites wouldn't come up. That would definitely skew the 0.1% number.
Not making a huge argument here, I just started down that analysis path out of curiosity and figured I'd share it.
What percentage of websites serve 1 mil or more requests per hour, .01%? .001%? Meaning Rails is going to be performant enough for 99.9+% of projects, and for those projects it would have been a mistake to trade dev time for performance you’ll never need.