I am creating a character for a video game. I have searched online how to make a leather texture for my characters hand. As I finished it, I have noticed that Blender's performance had dramatically changed for the worse; which is very slow. That's normal since I have a low end laptop, however, I don't know if using textures like this will affect a game's performance. My question basically means this, if I was to create a complex texture on my model which has reasonably low poly count, and I was to put the character in a game, would it make the game impossible for people to play lower end devices such as low spec computers?
You should be looking into how to "bake textures."
Probably, the material you created for your character's hand was a procedural material. A whole bunch of nodes, spaghetti-stringed together, right?
Procedural materials impact performance. But procedural materials are rarely (almost never) used in games. Instead, you bake the data used by your procedural material to images, so that they stop being procedural. At that point, they stop being procedural, and they don't impact performance any more than any other set of images would.
Not everything can be baked. View dependent stuff, like sheen, specular doesn't bake, not directly, but you can bake a sheen map or a specular color map or a roughness map. If you need something procedural in a game, you need to write a shader for that material, and your Blender material isn't relevant to that except for as prototyping.