What you need

  1. The latest version of Minecraft. A fresh install is recommended.
  2. The latest version of Optifine (link). Make sure it matches the Minecraft version you installed.
  3. The latest version of Continuum (link). That's what enables the mapping of Realistico.

Realistico needs certain settings to be displayed correctly.
I made a zip with all the files you need: download it here (updated on the 4th of February, 2019).
You should put these files in their folders after following the screenshots below.

The texture pack will be displayed as "incompatible" and a warning message will pop-up: click yes.

Now drag and drop the shader pack (Continuum-2.0.2.zip) into the shaderpacks folder.

Ok, now let's go back and fix some rendering settings.

Now, close the game and open the minecraft directory. On Windows, it's usually under AppData/Roaming (you can go to AppData by searching for %appdata%).
Now, unzip the archive I made: you'll find some txt filex in it.
Copy those files in the right folders: if some files are already there, replace them. Also, make sure that the shader pack is a a zip and don't rename it.
Here's an example of the folder structure:

At this point, the game should look like in the following screenshot.

For reference, I'm using a 3.40 GHz CPU, 16GB of RAM and a GTX 1080 on a 1080p monitor.
I get a consistent 30 FPS in full screen (1920x1080).
An SSD is more likely to give you these type of performance. If you're looking for a fast SSD on a budget, I recommend this one. That's the one I use, and it's amazingly fast.

If you're looking to upgrade your computer, I recommend the following components: CPU, GPU, RAM, SSD.

If you experience any kind of problem, you can fix common issues and read answers to common questions.