Let me help you abit here:
Go buy or download (Cracked): WinAVI MP4 Converter.
Install all the Real Codec, AVI and what ever codec you want on your computer..
Launch the program, and you can convert any format video (as long as you comp can play them) into mp4 format.
You can either choose a size format for your phone or use default.
COnvert and put it in your phone.
I gurantee 100% plus chop no LAG!!!!!
The reason why it lag is because of the converter that you are using is not up to standard. Try this and you will find it every phone in the world will be able to play any video converted by this.
Been doing this for iphone, Win mobile, android Sony ericsoon phone