Sasuke only used the sharingan to copy Lee's basic taijutsu forms so that he could replicate them. He had to work to master those forms, but it didnt take nearly as long as Lee did because Sasuke is naturally talented, which allowed him to grasp things much faster than normal. He also had to train his body so that he could physically achieve this speed using the taijutsu forms that he had mastered previously (NOTE: Again, he didn't master taijutsu in general, but was able to replicate Lee's mastery of the forms of taijutsu that Lee uses)
While this training method has its benefits in that Sasuke was able to replicate Lee's great speed within a month, the downside is that he cannot use this speed for nearly as long as Lee can because he doesnt have Lee's physical build and stamina. It requires great dedication to the physical arts to both achieve and maintain this level of speed, an example of what only hard work alone can achieve.