I actually think he is a pretty decent player but like many can see his clear lack of pace is an issue. A lack of pace against pacy & skillful attackers can make the most technically gifted defenders look error prone and amaturish.
Like others have highlighted, improving his posistional awareness and reading of the game will help compensate for the lack of pace but against in-form opponents (unless yuh is Bekenbauer) even that might not be enough. Have a look at Man Utd against Barcelona in 1994; even with the hallowed Bruce and Pallister at the back, Romario, Stoickov et al rip them tuh shreads in that game. They were a great partnership, quality defenders and read the game extremely well but were not known for Des Walker like speed. Barcelona exploited that.