NEW Nigerian President Bola Tinubu has moved at lightning speed in his first month in office, implementing a raft of radical changes aimed at finally unleashing the full potential of Africa's sluggish economic giant. But while his rapid headway on reforms has wowed investors, some analysts, observers and business leaders warn that bigger challenges await and question whether the 71-year-old - viewed by many as part of Nigeria's old guard - is really the man to take them on. "There has been a big policy shift (but) I think there are things that investors would like to see happening to really be convinced…