Why computer take long time to open huge text files? Im not sure if this the right place to ask but I'm wondering why a computer takes forever to open huge text file like 1GB but if I open a video file (mkv) for example with 10GB like a movie it will be opened very fast.

Why the computer doing this ? What is the difference between the two operations?
The difference would be between loading the complete file into memory and reading the file part-by-part as you need each part.

If I open a file in some applications I am asking for it to be fully loaded into memory all in one go.

If I play a movie file or DVD in most player applications the applications will be structured to just fetch data as it needs it, not all at once.

There are different ways to open files and devices in operating systems and they are not all the same.