Lehi is already very fluent in pretty much everything he does on the field.
But... I was a little bit concerned in the Tonga vs Australia match when he couldn't chase down Tom (who had the ball in hand) at the end of the year. It's not a huge concern as Tom is still an elite player. However, if he doesn't have the speed to do that at 19 years old and a bit over 80kg, then to me it would suggest that maybe (and this is just my opinion) his long-term future may not be as a winger or fullback.
Players like Dylan Edwards don't have exceptional size or speed by NRL standards either, but they have a huge work-rate and are surrounded by players that do have size and speed. As far as Lehi goes, we'll cross that bridge when we get to it.