When I tried to extract (used right click and extract) a 2Gb ISO file it took a lot of time. I tried to mount the ISO using
-o loop and its works fast. Why is it so?
You should clarify what you mean by extracting but as far as I understood:
in the first case you copy each file and folder out of the ISO file. By making a copy you need to read everything and write everything to a different location
mounting just makes the content available but does nothing else (no copy thus no read/write). The content is available as part of the file system but is read only when you make an operation on a file.