Morning, @saikat903852 and welcome!
A little difficult to know what you mean with “open/process” 
But if I open a terminal, and type in the command
rawtherapee _LGH0064.NEF
The image will open up on the monitor in about 1.53 seconds.
The .NEF comes from this thread: Strong gels, low key photo - #14 by XavAL
However, I am not using a laptop, but a Ryzen 9 3900X CPU with a good GFX
and lots of RAM. But at least that would give you a figure to compare with.
Have fun!
Claes in Lund, Sweden