Generally when ever you are trying to watch a video , your video player will take some time to load the required libraries to play the video and video can be played properly after proper decoding. So what ever the short lagging you are feeling due to Loading of video libraries and decoding of your video to play.

while doing the any data operations CPU will consume its maximum free memory. So when ever your Transmission had completed the download it needs to check the data before displaying the status as Completed.

I think for those two situations that explanation can help you to understand the issue,:)