Remember that there is a computer in computer science. It is important to understand what your code runs on and the effects that this has, this isn't magic.
Computers are so fast that it can be difficult to understand which operation is a quick operation and which is a slow one. Everything appears instant. In fact, anything that happens in less than a few hundred milliseconds is imperceptible to humans. However, certain things are much faster than others are, and you only get performance issues at scale when millions of operations are performed in parallel.
There are various different resources that can be accessed by an application, and a selection of these are listed, as follows: