I finally got my 3ds xl installed with the capture card! I'm really thankful for Merki to help fixing my console as I didn't know it's a refurbished version (bought from second hand marketplace). Though I have no clue what actually causes the slow down but I'm glad that the installation could proceed as normal.
The driver installation part is a bit trial and error on my side because there are a few files inside the driver download folder. I misunderstand the 'reboot system' part as in restarting my pc and realize it actually refers to my 3ds (⌒_⌒;) At last I got the software running! The game runs smooth and displays real time but for some reasons the music sounds a bit lagging every once in a while during gaming. Maybe it's my pc getting old.
I cannot find the capture software (3dsCapture.exe) inside the driver download folder (Keity_old3dsview). Maybe it's only available for newer models?
TL;DR feedback: Merki is really really nice! A very friendly seller that really helps a lot to answer my questions & even help to fix issue of my old console!! Thank you so much Merki!