Questions about watching movies on my PSP?

Well i used to by the UMD disks to watch movies on my psp but its pointless when i want to watch a movie on my tv and i bought it for my PSP not a dvd anyway i have a program called DVD fab that lets me convert dvds into PSP format my question is if i get like a 4gb memory stick and put the movie on there will it play like a normal DVD or will it be laggy because of being on the memory not on a disk?