Do the videos use external subtitles e.g. seperate .srt or .ssa or other similar types of files in the same folder as the avi? If so, it's probably a codec/filter problem that's causing the lag, and probably need settings adjusting. If not, then as it's avi, the subs will be "hard-subbed" onto the video, so there's nothing you can do about it other than complain to whoever subtitled the videos.