Probably because you offer no evidence whatsoever, a typical characteristic of rants?

I could entertain an argument that Rails is a poor technology choice. It certainly makes it easy to shoot yourself in the foot without sufficient experience, despite that being a conscious tradeoff towards ease of learning. But Ruby? What, precisely, is wrong with Ruby?

GitHub, Airbnb, Shopify, Indiegogo, Kickstarter, Urban Dictionary... the list goes on and on and on. Having the hardline opinion that Ruby or rails is slow is plain ol’ ignorance. You can build highly performant apps in Ruby and Rails, you just need to know what to optimize. The same applies to any other language or platform. Facebook is still humming along on PHP.

Ruby is slow for humans too. Code intelligence/IDEs are hamstrung, thanks to the constant possibility of run-time monkeypatching. You basically are stuck with grep. It's fine if you have 30 engineers all familiar with Ruby and the codebase, not so great with 100s of engineers trying to figure where things are. Things get worse if your engineers embrace writing DSLs in Ruby and you now have to debug a mini language-within-a-language.

The dirty secret at large companies is that the majority of the codebase sucks and is maintained by a legion of new grads who are doing their best to ship code before deadlines. Ruby is not the answer.

Probably fair, but certainly not unique to large companies using Rails.

I like Ruby quite a lot. Rails can pound sand, but Ruby is one of the better inventions in the software space. But it is and always is slow on every axis you could want save "developer productivity" and arguing against it is arguing against the tide.

It is not "ignorance" to assert this; it is data. It very well may be fast enough for your use case, and I don't know many people who would say it's not, but it is not and never fast.

(JRuby might improve this, but since few people use it, MRI is more specifically under discussion.)