But what do you mean, give me a real example. You loaded too many active_records in memory and it's using a ton of ram? Did you try pluck, batches or even skipping active_record and using a raw query?
Unless you really need to scale for a ton of users, you don't have to go crazy to get decent performances out of rails/ruby. How many requests/sec are we even talking about here?
Doesn't matter if SQLite is now viable with rails, no legacy rails app is using it and it's not like you're going to wake up one morning and migrate your production db from postgres, mysql, sql server or oracle just because you felt like it.
In theory the language is slow, in practice it doesn't really matter because the db is much slower unless you're github or twitter and you really need to scale.
When you choose ruby, you trade faster dev time for slower runtime. I am OK with this trade-off 99% of the time. My dev time costs way more than a couple ms lost on my production server.
There are plenty of businesses that have under 10k users and can live perfectly well with http requests around 500-1000 ms. When there are performance issues, 95% of the times they come from the database, not the language.
Modules (mixins) are objects, classes are also objects (of type Class).
The Kernel module is included in the Object class, which means its methods are available to every Ruby object and can be accessed from virtually any scope, including the top-level (global) context where self is an instance of Object called "main."
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Unless you really need to scale for a ton of users, you don't have to go crazy to get decent performances out of rails/ruby. How many requests/sec are we even talking about here?