That would happen if Transmission is writing lots of data to the disk or reading lots of data while you are watching in VLC.
And god...why does everyone bring up Plex when 'video' and 'lag' appear in the same post? Plex is a bloated, 10-click way (about 100 clicks if it's your first time) to watch a single video. Try Movist.
Thank you for the replies!
I'm watching movies as the dl is going, not when its finishing up. I love VLC but its very annoying not being able to set my torrents to download while enjoying a show/movie.
I will try both Plex and Movist and see how that goes.
I hope vlc or transmission can fix this problem.. or is it just that Transmission lags anything?