Any web application is going to be almost entirely network and database-bound, not CPU-bound. CPU-bound tasks are stuff like scientific computing, video encoding, the Ruby interpreter and Crystal compiler themselves, etc.

The advantage of multithreading in a web application is not "using all cores" but rather that the application can spawn a thread to "asynchronously" wait for a response from the network or database while it continues doing other work in the main execution thread, instead of "synchronously" blocking all work while it waits.

In other words, web applications leverage concurrency with their use of multithreading, not parallelism.

I will caveat that having more cores can give concurrent applications a small performance improvement, because even though each thread is only using a CPU core at around 0 to 15% (in short bursts) of capacity, switching between two threads on the same core is a context switch, a fairly expensive operation. More cores means fewer context switches.

But, unlike with parallelism, making use of more cores is not the reason concurrency offers performance benefits. ~99% of the performance benefit of concurrency is realized on even a single-core machine, whereas parallelism by definition requires multiple cores to offer a speedup. Concurrent applications such as web servers (e.g. Apache, Nginx) typically spawn hundreds or even thousands of threads per CPU core.