Why computer take long time to open huge text files?
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.